User & has no user master record

What causes this issue?

You attempted to enter a user with no user master record in a user
group.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Define a user master record for the user. Then, you can enter the user
in user groups.

Error message extract from SAP system. Copyright SAP SE.