Replenishment &1 stopped because MRP group not found for material

What causes this issue?

The in-stock records cannot be processed for the purchase order or the
production order. The system could not find an MRP group for the
material.

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?

Enter an MRP group in the material record.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.