Material &1 in FFF class &2 exists in MRP set &3

What causes this issue?

Material &V1& cannot be deleted in the FFF class &V2&.

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?

Maintain the <DS:GLOS.3608C3DABA271348E10000009B38F91F>MPN-MRP set</>
for the FFF-class &V2&. Delete material &V1& from MPN-MRP set &V3&.
Maintain the FFF-class

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.