Unless your products are custom. You need to use the barcode filed with a GTIN. GTIN is your UPC for a North American product or EAN for European....sect depending on where the product is made around the world. There is no way getting around this. You need to use the barcode field or have a customer product. We have a few drop ship clients and they still put in the GTIN.
I've reached out to my suppliers and they t old me they don't have a GTIN, so I don't know what to do at this point.
This Google support page shows what different GTINs are around the world. Not sure what you are selling, but most products have one.