Try this:
nomis@vs1:~$ /opt/plesk/php/7.2/bin/php /usr/lib/plesk-9.0/composer.phar install
Don't forget to install / enable PHP 7.2 or use PHP 7.1 instead.
Plesk Installer: Tools & Settings -> Update and Upgrades -> Add/Remove Components
or
CLI: plesk installer --select-release-current --install-component php7.2
Also don't use root account in Plesk VM for this ... use system user for your domain (webspace) instead. You might have problems with file permissions.
Domains -> xxx.com -> Web Hosting Access -> System User, Access to the server over SSH /bin/bash.