Can't install drush with composer

Question

When I run...

composer global require drush/drush:dev-master

...I get...

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - Can only install one of: consolidation/output-formatters[3.1.12, 2.1.3].
    - drush/drush dev-master requires consolidation/output-formatters ^3.1.12 -> satisfiable by consolidation/output-formatters[3.1.12].
    - Installation request for drush/drush dev-master -> satisfiable by drush/drush[dev-master].
    - Installation request for consolidation/output-formatters (locked at 2.1.3) -> satisfiable by consolidation/output-formatters[2.1.3].

---

My PHP version is 5.6.11.

Answer 1

The solution in my case was to run...

composer global update
composer global require drush/drush:dev-master

Previously, I had been running composer update but needed to run composer global update.

Answer 2

Assuming this is a Drupal 8 question; The recommended method of installing Drush is to now install it as a dependency of your project, not a system application.

composer require drush/drush

This will of course install Drush 9. If for some reason you need Drush 8 then other methods are available. But really not having to get the sysadmin involved in order to upgrade Drush is a relief.