Check the README.txt of the S3 File System module. Contains a couple of instructions that are helpful.
In the unlikely circumstance that the version of the SDK you
downloaded causes errors with S3 File System, you can download this
version instead, which is known to work: https://github.com/aws/aws-sdk-php/releases/download/2.6.3/aws.zip
IN CASE OF TROUBLE DETECTING THE AWS SDK LIBRARY:
Ensure that theawssdk2 folder itself, and all the files within it, can be read by your webserver. Usually this means that the user apache (or _www on OSX) must have read permissions for the files, and read+execute permissions for all the folders in the path leading to the awssdk2 files.
The link to the aws-sdk-php on the AWS installation page will allow you to download the latest version (3.4.1).
On my case, I had to download the 2.6.3 version of the SDK that is mentioned on the README.txt.
You might want to check at admin/reports/dblog if there are any errors related to loading the SDK.
I found that sometimes the libraries info gets hard cached and that is why this module has the
_s3fs_load_awssdk2_library(). Take a look of this:
This function is a replacement for calling
libraries_load('awsdsk2'). It's needed because
libraries_load() caches failures to load the library, meaning that temporarily having a bad setup (e.g. nonexistent or unreadable files in the awssdk2 folder) can lead to the library being permanently unable to be loaded, even after the bad setup is repaired. This can only be remedied by clearing the full site cache.
TD;LR: sometimes you will need to refresh the libraries cache for the SDK, if that is the case, the
_s3fs_load_awssdk2_library() can be invoked by going to
admin/config/media/s3fs/actions and use the "Refresh file metadata cache" button option. Or also you can use the Drush command