The receiving storage location &1 should be assigned to MRP area &2

What causes this issue?

An MRP area of business type contract packager must have a receiving
storage location assigned to it . The same storage location should also
be explicitly assigned to the MRP area in customizing via the <AB><LB>
<LS>Assign Storage Locations. </></></>

System Response

The error message is shown if the receiving storage location of the MRP
area has not been assigned in this dialog.

How to fix this error?

Assign the receiving storage location at first and then assign all the
other storage locations that belong to this MRP area.

Procedure for System Administrators

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.