Generally Amazon will need to know your Product Identifier (GTIN/UPC/EAN/ISBN) in order for you to sell.  This is usually located in your Barcode field in Shopify.

Amazon Channel by Codisto will automatically this field to the GTIN field in Amazon so usually there is nothing more to do.

However, if for some reason you have Product Identifiers in a field other than your Barcode field you will need to change the mapping.  You do this by clicking the field as part of the checklist flow when you first connect Amazon which takes you to Attribute Mapping.

You can also navigate there via "Listings".

To change your field mapping, go to 'standard attributes' section, simply open the GTIN field dropdown and select the field in which your product identifiers are stored in Shopify.

If you don't have Product Identifiers in Shopify (or not for all products), the alternative to mapping is to enter them in the Product Identifer field in the Single Edit listing mode (found in 'Listings').

*If you have a GTIN exemption from Amazon or are only creating Offers and send the ASIN of that Offer to Amazon (enter in ASIN field in 'Listing' see screenshot above) it is possible to not send a GTIN to Amazon.