&1 refers to ERP WBS element &2 in system &3

What causes this issue?

You have tried to assign a WBS element &V1& to the marketing element.
The marketing element, however, already has an assignment to WBS Element
&V2& in the ERP-System &V3&.

System Response

INCLUDE NA_FUNKTION_NICHT_M+GLICH OBJECT DOKU ID TX

How to fix this error?

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

Error message extract from SAP system. Copyright SAP SE.