The following is an error involving Item Package Quantity that might get returned:

The SKU data provided conflicts with the Amazon catalog. The item_id value(s) provided correspond to the ASIN ______, but some information contradicts with the Amazon catalog. The following are the attribute value(s) that are conflicting: item_package_quantity (Merchant: '_' / Amazon: '_'). If your product is this ASIN, then modify your data to reflect the Amazon catalog values. Else, check your value(s) for item_id are correct

Item package quantity refers to the number of pieces that might come with one SKU. It's intuitive to think of it as one item but the item package quantity is the amount of components that make it up. To fix this error simply input the number Amazon wants it to be. If you are sure it's not that number then chances are your item is tied to a different ASIN.