WBS elements could not be included. Check your entry

What causes this issue?

The selected WBS elements can not be included.
There could be several reasons for this error:
You did not enter an identifier for the WBS elements.
You assigned the same identifier for several WBS elements.
A level is missing from the hierarchy you want to include.
You used a locked structure indicator as an identifies for a WBS
element.

System Response

INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

How to fix this error?

Choose <LS>Check</>. You will get an error log in which the individual
errors are displayed.

Error message extract from SAP system. Copyright SAP SE.