A fatal error occurs when installing Composer dependencies with Drush

Composer Manager depends on the Composer Drush extension to install the Composer tool.

Possible cause: Outdated Composer

If you run drush composer-manager commands and see errors like the following, try updating your Composer Drush extension.

[RuntimeException]
Could not load package...
[UnexpectedValueException]
Could not parse version constraint...

To resolve this issue, delete the current version of the Composer Drush extension and reinstall it by running the following commands ( 8.x is the correct version):

rm -Rf ~/.drush/composer
drush dl composer-8.x

Then run the following command inside of your Drupal installation:

drush composer-manager install