How do I resolve this installation error?

Question

I am getting the following error while installing Drush with composer require drush/drush.

Using version ^8.1 for drush/drush
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- drush/drush 8.1.0 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.1 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.2 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.3 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.4 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.5 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.6 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.7 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.8 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- drush/drush 8.1.9 conflicts with drupal/drupal [No version set (parsed as 1.0.0)].
- Conclusion: don't install drush/drush 8.1.12
- Conclusion: don't install drush/drush 8.1.11
- Conclusion: don't install drush/drush 8.1.10
- Conclusion: don't install drush/drush 8.x-dev
- Installation request for drupal/drupal [No version set (parsed as 1.0.0)] -> satisfiable by drupal/drupal [No version set (parsed as 1.0.0)].
- Conclusion: remove phpdocumentor/reflection-docblock 3.1.1
- Installation request for drush/drush ^8.1 -> satisfiable by drush/drush [8.1.0, 8.1.1, 8.1.10, 8.1.11, 8.1.12, 8.1.2, 8.1.3, 8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.8.x-dev, 8.x-dev].
- Conclusion: don't install phpdocumentor/reflection-docblock 3.1.1
- drush/drush 8.8.x-dev requires phpdocumentor/reflection-docblock ^2.0 -> satisfiable by phpdocumentor/reflection-docblock [2.0.0, 2.0.0a1, 2.0.0a2, 2.0.0a3, 2.0.1, 2.0.2, 2.0.3, 2.0.4, 2.0.5].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.0, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.0a1, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.0a2, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.0a3, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.1, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.2, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.3, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.4, 3.1.1].
- Can only install one of: phpdocumentor/reflection-docblock [2.0.5, 3.1.1].
- Installation request for phpdocumentor/reflection-docblock (locked at 3.1.1) -> satisfiable by phpdocumentor/reflection-docblock [3.1.1].

Installation failed, reverting ./composer.json to its original content.

Answer 1

Had the same problem started from D8.4beta1.

Solved by switching to Drush 9 with:

composer require drush/drush:^9

Answer 2

The fix was downgrading phpdocumentor/reflection-docbloc to 2.x with composer require phpdocumentor/reflection-docblock:^2.0.

./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 1 update, 0 removals
- Installing dflydev/markdown (v1.0.3): Downloading (100%)
Drupal\Core\Composer\Composer::vendorTestCodeCleanup
- Updating phpdocumentor/reflection-docblock (3.1.1 => 2.0.0): Downloading (100%)
Drupal\Core\Composer\Composer::vendorTestCodeCleanup
Package dflydev/markdown is abandoned, you should avoid using it. Use michelf/php-markdown instead.
Writing lock file
Generating autoload files
Drupal\Core\Composer\Composer::preAutoloadDump
Drupal\Core\Composer\Composer::ensureHtaccess

Then install Drush with composer require drush/drush.

Using version ^8.1 for drush/drush
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 12 installs, 0 updates, 0 removals

Answer 3

I was able to reproduce the error, using the following commands:

composer create-project drupal/drupal 8 ^8.3
cd 8
composer update
composer require drush/drush

The last command gave the following output.

Using version ^8.1 for drush/drush
./composer.json has been updated
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

Problem 1
- drush/drush 8.1.0 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.1 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.2 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.3 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.4 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.5 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.6 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.7 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.8 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- drush/drush 8.1.9 conflicts with drupal/drupal[No version set (parsed as 1.0.0)].
- Conclusion: don't install drush/drush 8.1.12
- Conclusion: don't install drush/drush 8.1.11
- Conclusion: don't install drush/drush 8.1.10
- Conclusion: don't install drush/drush 8.x-dev
- Installation request for drupal/drupal No version set (parsed as 1.0.0) -> satisfiable by drupal/drupal[No version set (parsed as 1.0.0)].
- Conclusion: remove phpdocumentor/reflection-docblock 3.2.0
- Installation request for drush/drush ^8.1 -> satisfiable by drush/drush[8.1.0, 8.1.1, 8.1.10, 8.1.11, 8.1.12, 8.1.2, 8.1.3, 8.1.4, 8.1.5, 8.1.6, 8.1.7, 8.1.8, 8.1.9, 8.8.x-dev, 8.x-dev].
- Conclusion: don't install phpdocumentor/reflection-docblock 3.2.0
- drush/drush 8.8.x-dev requires phpdocumentor/reflection-docblock ^2.0 -> satisfiable by phpdocumentor/reflection-docblock[2.0.0, 2.0.0a1, 2.0.0a2, 2.0.0a3, 2.0.1, 2.0.2, 2.0.3, 2.0.4, 2.0.5].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.0, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.0a1, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.0a2, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.0a3, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.1, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.2, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.3, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.4, 3.2.0].
- Can only install one of: phpdocumentor/reflection-docblock[2.0.5, 3.2.0].
- Installation request for phpdocumentor/reflection-docblock (locked at 3.2.0) -> satisfiable by phpdocumentor/reflection-docblock[3.2.0].

Installation failed, reverting ./composer.json to its original content.

In fact, composer update output the following (among other lines):

• Updating phpdocumentor/reflection-docblock (2.0.4 => 3.2.0): Downloading (100%)

I tried also using composer require drush/drush dev-master and/or composer create-project drupal/drupal 8 8.4.*@dev but all the combinations caused the same error messages.

I was able to install Drush 8.1 with Drupal 8.3 using the following commands.

composer create-project drupal/drupal 8 ^8.3
cd 8
composer require drush/drush
composer update

In this case, Composer doesn't install anymore phpdocumentor/reflection-docblock 3.2.0, but 2.0.5.

• Updating phpdocumentor/reflection-docblock (2.0.4 => 2.0.5): Downloading (100%)

drush status outputs the following.

 Drupal version         :  8.3.5
 PHP executable         :  /usr/bin/php
 PHP configuration      :  /etc/php/7.1/cli/php.ini
 PHP OS                 :  Linux
 Drush script           :  /home/kiamlaluno/8/vendor/drush/drush/drush.php
 Drush version          :  8.1.12
 Drush temp directory   :  /tmp
 Drush configuration    :
 Drush alias files      :
 Drupal root            :  /home/kiamlaluno/8
 Drupal Settings File   :  MISSING

Answer 4

I think easy way is via

composer install
composer require drush/drush

Answer 5

Had similar issue with drush installed twince onto both /usr/lib/ and the composer webroot

• composer remove drush
• locate more drush installations (on my debian was "which drush")
• remove those from your system
• re-install drush following https://docs.drush.org/en/8.x/install/ or https://docs.drush.org/en/8.x/install-alternative/

Answer 6

Errors like that mean that the installing package requires other dependencies, so you can always use the -W flag to install the package with dependencies.

composer global require drush/drush:8.x -W