Structure indicator & ... is locked

What causes this issue?

Key &V1& is not allowed for creating new objects.
In this situation, you cannot create any new standard project
definitions, standard WBS elements, project definitions or WBS elements
with this key.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Use a different key for the project you want to create.
INCLUDE 'NA_ALLG_VORGEHEN' OBJECT DOKU ID TX
You can define the key for project coding in Customizing.
Check your settings for project coding in Customizing. Delete the lock
indicator, if necessary.

Error message extract from SAP system. Copyright SAP SE.