Purchase requisition &1, item &2, has a different material group

What causes this issue?

This item has a reference to a purchase requisition. However, the
material group of the purchase order item is different from the
material group of the purchase requisition item.

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?

Check the item's material group.
You may need to change the reference to refer to a different purchase
requisition.

Error message extract from SAP system. Copyright SAP SE.