User interface predefined, because there is no definition in Customizing

What causes this issue?

A profile for single screen entry has not been defined as a standard
profile, neither has the current user been assigned a profile in the
CORUPROF parameter in the user defaults.

System Response

For single screen entry, the system uses a firmly defined interface.

How to fix this error?

If you do not want to use this firmly defined interface for entering
data, you have the following options:
Define a profile for single screen entry as a standard profile.
<DS:TRAN.OPK0>Execute function</>
Assign a profile for single screen entry to the CORUPROF parameter in
the user defaults for the current user.
<DS:TRAN.SU3>Execute function</>

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.