Manual maintenance of the delivery relationship is not possible

What causes this issue?

For the selected combination of MRP area and material or for the MRP
type &V1& assigned to this combination, method '&V2&' is set in
Customizing for determining delivery relationships. This method does not
permit the delivery relationship to be maintained manually.
Either no determination of the delivery relationship is set for the
selected type of recipient (MRP area) (method ' '), or a reference to
another delivery relationship is used (method 'R').

System Response

You cannot maintain the delivery relationship manually or recreate it.

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.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.