@boris789 is correct. Recently, Amazon inteoduced a new policy prohibiting the updates to Generic ASINs by third parties. This is to prevent sellers from taking control of generic listings and hijacking them into updating them to their own trademarked brand to prevent others from joining the listing:
“If you try to edit or add offers on another seller’s generic product, you’ll receive an error message in feeds or in the one-to-one listing process. You will then be guided to create a new product in the Add a Product tool.”