Initial element cannot be deleted

What causes this issue?

When you entered WBS element &V1& you selected that portion of the WBS
which belongs to this WBS element. In this case, you cannot delete WBS
element &V1&.

System Response

INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

How to fix this error?

Call up the transaction again and enter only the project definition,
(this automatically selects the entire WBS), or simply enter a WBS
element which is superior to WBS element &V1&.

Error message extract from SAP system. Copyright SAP SE.