WBS element & is not an account assignment element

What causes this issue?

The activity or the activities that you want to assign to a WBS element
are in a network which has account assignments at the activity level.
In this situation, the WBS element must be an account assignment element
Because the WBS element &V1& is not defined as an account assignment
element, you cannot make any actual or open item postings.

System Response

INCLUDE 'NA_ALLG_NOVERARBEITUNG' OBJECT DOKU ID TX

How to fix this error?

Correct your entry by entering a WBS element that has been defined as an
account assignment element.
If you really want to post WBS element &V1&, the system administrator
or another authorized person, such as the person responsible for the
project, must mark it as an account assignment element.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Procedure for the system administrator:
Mark WBS element &V1& as an account assignment element.
ENDIF
<DS:TRAN.CJ02>Proceed</>
ENDIF

Error message extract from SAP system. Copyright SAP SE.