You cannot change the FFF class, & belongs to a MPN-MRP set

What causes this issue?

The &V1& material is assigned to an MPN MRP set. You must not change
the assignment to the old &V2& FFF class, while the material is
assigned to an MPN MRP set.

System Response

Processing terminates.

How to fix this error?

Remove the material from all MPN MRP sets, or do not change the FFF
class.

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.