Unable to install or update Plesk on the CloudLinux server

Symptoms

Installation or update of Plesk on a CloudLinux server fails with the following error:

PLESK_ERROR: There was an error communicating with CLN.
CLN support will be disabled.
Error communicating with server. The message was:
com.cloudlinux.clos.domain.exceptions.xmlrpc.ClnXmlRpcException: (=_=) Invalid System Credentials. Cannot parse request or identify server. Please, run registration again.

Cause

The CloudLinux OS license is missing.

Resolution

Install the CloudLinux OS license. In case of CloudLinux license related inquiries, contact Cloudlinux team for help.