https://userapps.support.sap.com/sap/support/knowledge/en/3741451
3741451 - Return and Credit Memo Not Supported for Free-of-Charge Item Category (CBXN/CRXN) in SAP...
It is not possible to create a return (REB/ROB) or Credit Memo Request (CRB) with reference to a billing document (F2) containing free-of-charge item category...