Processing using function module -> ID remains blank

What causes this issue?

When creating a process code for which processing is designated to be
via function modules the identification is maintained in table TBD52
and remains blank here. It is only referred to for display.

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?

First save your process code and then maintain table TBD52 via the
programmed jump from the transaction or via <DS:TRAN.BD67>transaction
BD67</>.

Error message extract from SAP system. Copyright SAP SE.