This MySQL error can occur when you are trying to add user to database. Usually, the problem is on complete machine.
It looks something like this:
There was a problem adding the user: (XID 9bbt7j) Database Error: Can’t find any matching row in the user table
The solution is to check and see if skip-name-resolve is enabled in your /etc/my.cnf file on this system. If it is comment that line or delete it.
Restart MySQL through WHM/cPanel, or with command