FI-SL expert mode is active

What causes this issue?

You have activated the FI-SL expert mode in your user parameters
(<DS:TRAN.SU3>-></>).

How to fix this error?

You can execute procedures in expert mode that are normally prevented
by the system. For example, you can edit <DS:DE.SETCLASS>set classes</>
outside of the FI-SL application with the general set maintenance.
Note that improper actions performed in expert mode can lead to data
inconsistencies in the system. You should only use expert mode for
repairs.

Error message extract from SAP system. Copyright SAP SE.