SNC name and specified user/client do not match

What causes this issue?

You have used the Secure Network Communication (SNC) function to log
on. At the same time, you specified an R/3 user name. However the
external (SNC) name delivered from the external security system to the
R/3 System is not assigned to the specified user in table USRACL.

System Response

The system refuses the logon.

How to fix this error?

Log on with the permitted user name, or use the user maintenance
transaction (Transaction SU01) to assign the SNC name to the user.

Error message extract from SAP system. Copyright SAP SE.