You cannot delete/overwrite the assignments

What causes this issue?

One or more company codes are already assigned to controlling area &V1&
and account assignment objects (cost centers, orders, ...) already
exist in the controlling area. For this reason you are not allowed to
delete the existing assignments.
In cross-company code cost accounting you are only allowed to assign
one unassigned company code to controlling area &V1&. Once you save the
entry, this company code can no longer be deleted as an assignment.

System Response

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

How to fix this error?

You are not allowed to delete existing assignments. Contact your SAP
consultant.

Error message extract from SAP system. Copyright SAP SE.