When using the Plesk PHP Composer module, installing or updating dependencies sometimes throws a PHP memory limit error. This is due to Plesk not assigning enough memory to run the updates. To ensure that Plesk PHP Composer works, when in the composer screen, click on the "specify" button in the "Environment variables" section. Here you can set the composer memory limit. I would recommend setting it to unlimited by using the following variable:
COMPOSER_MEMORY_LIMIT = -1 (see attached screengrab)