Do not maintain the "Function module description" field

What causes this issue?

You did not maintain determination type 'C' (determination of customer-
specific function module). You entered a function module, however.

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?

In this case you should not maintain a function module. Delete your
entry.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.