Refer MOS notes:
Stuck Transaction Interface Sales Order Issues: An Error Occured While Relieving Reservations (INV_RSV_RLF_FAILED) [ID 280400.1]
Misc Issue Fails in Transaction Open Interface with Error: "An error occurred while relieving reservations" [ID 437292.1]
Ensure that you are not violating any existing reservation while performing subinventory transfer.
(e.g. you have subinventory level reservation, and you are trying to move the stock from that subinventory.
Or you have org level resevations, and you r doing subtransfer from Reservable subinvenotry to non-reservable subinventory.)
Basically you need to ensure that "Available to Reserve" (ATR ) quantities are any level (locaator/subinventory/lot/organization) should always remain positive.
If you are violating the rule above, then you will get this error.
Are you doing subxfr to move the stock from Staging to stores?
Did you check if Scrap subinventory is non-reservable?
if you have onhand Quantity of X and org level reservation for quantity of Y
and if you are subtransferring to non-reservable subinventoy,
system will allow you to transfer only (X-Y) quantity. if you transfer more than that system will give you the error.