Screen for coding can only be extended, not changed.

What causes this issue?

There already is a project definition or a WBS element for this key.
You can only change the coding under certain conditions.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

If you want to change the coding, you can only make alpha-numerical key
longer.

Error message extract from SAP system. Copyright SAP SE.