/user/bin/env: 'php' : No such file or directory

My sites were all built manually without Composer - simply by downloading modules and themes and installing them that way.

I'd like to use Composer with my sites now, so:

I've downloaded and installed Composer on my computer.

I've installed the Plesk extension PHP Composer on my Linux server.

I log into Putty SSH and go to the root directory of any of my sites and run any composer command and I get the following:

/user/bin/env: 'php' : No such file or directory

Is there something I need to do to fix this issue?