The following error appears on attempts to add a Plesk server in Plesk 360 > Servers > Servers:
PLESK_ERROR: The connection to the server server.example.com cannot be established correctly. Plesk version is not supported (below 18.0.37) or Plesk is misconfigured.
The current Plesk version on the server is less than 18.0.37
Plesk 360 platform allows adding servers with Plesk version 18.0.37 or above.
Update Plesk to the latest version and, then, try to connect the server once again.