clarkwinkelmann Thank you very much.
The problem is fixed now. The solution is simple (only now).
In SSH I create a file called .bash_profile with the following command:
touch .bash_profile
I then edit the file to apply the desired PHP version with the following command:
nano .bash_profile
and paste this code (for php7.3):
export PATH=/opt/plesk/php/7.3/bin:$PATH
Save it with CTRL+O, close SSH.
Thank you very much clarkwinkelmann, without your intervention I would not have succeeded.