If you are trying to do a bulk upload and get the error message Upload Failed: Identifier is Invalid what you should do is check your bulk file and see if you have a column for both GUID and SKU like as follows: 



Having both these identifiers is incorrect if you are trying to edit or relist in bulk. If products already exist and you are trying to bulk edit or relist, then all you need is either the SKU or GUID. Please keep in mind, it is always best practice to use the GUID as your identifier in bulk files.