WBS element & has been assigned to an incorrect controlling area

What causes this issue?

WBS element &v1& is assigned to another controlling area than the
grouping WBS element. Thre WBS elements within a grouping WBS element
group have to belong to the same controlling area.
In the case of grouping WBS elements whose validity depends on the
plant/MRP group, the controlling area of the grouping WBS element still
has to be the same as that of the plant.

System Response

INCLUDE NA_ALLG_NOVERARBEITUNG OBJECT DOKU ID TX

How to fix this error?

Check the controlling area of the grouping WBS element (in the project
definition). Make sure that the WBS elements that are assigned to this
grouping WBS element have the same controlling area.

Error message extract from SAP system. Copyright SAP SE.