The error "guid:51 - Value of SKU or GUID may not be modified unless ID is identifier" might be returned if you are doing a bulk edit and you have both your GUID and SKU as columns in your .csv:

If you have a bulk edit set up similar to the above picture, you will get this error returned because it looks as though you are trying to update the GUID. Whether or not that is your intention, the only time a SKU and GUID can ever be updated is if you have ID as your identifier column.