Question
Why is it impossible to log in to Plesk with system user credentials from the Domains > example.com > Hosting & DNS tab > Web Hosting Access menu?
Answer
If corresponding permissions are granted, system user accounts are used to serve FTP/SSH (on Linux) or RDP (on Windows) connections. They are not designed to be used for logging in to Plesk.
To find or create users that can be used to log in to Plesk navigate to Subscriptions > example.com > Users.