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

System Response
INCLUDE 'NA_ALLG_NOFUNKTION' OBJECT DOKU ID TX

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