Project & already exists. (You cannot use this key)

What causes this issue?

A project has already been creating using the key which you entered.
You cannot enter any more coding for a project definition or a WBS
element using this key.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Enter a coding for the key which hasn't been used yet.

Error message extract from SAP system. Copyright SAP SE.