According to the PhpSpreadsheet Doc it\'s neccessary to install it with composer. In my case I just have a webspace without Terminal but Plesk. Is it anyway possible to use
Have you considered installing composer locally, running it as instructed, and then just sending the generated files to the server via normal FTP? You install composer on your computer and run it there, then upload via FTP/SFP...
Composer is not "necessarily" intended to run live while the user is downloading the pages, it is intended to be run "mainly" on your local computer and generating the files and dependencies that later on you upload to the server. It can update the files on your server, but that is a convenience not a necessity.