Fix your Composer
My clients websites are running PHP 5.6.x and 7.0.x.
My clients have pushed me to work over new-year due to the dramatic change to PHP 7.x, but your system is still stuck in PHP 5.5 (possible since are not "4D thinkers" and leave Composer not working for us, that makes you the bottleneck, please fix it...
What do you mean “your system is still stuck in PHP 5.5”? Plesk support PHP versions 7.x and latest 7.3 even. I suppose that your Plesk site is still uses PHP 5.x version. Just switch it to 7.x version and repeat your composer action. If it not help – ask Plesk Support Team assistance of community assistance on https://talk.plesk.com