In most cases, products must have a Global Trade Item Number (GTIN), such as a UPC, an ISBN, or an EAN. Amazon uses these product IDs to identify the exact item you’re selling. If you match a listing, you won’t need to provide a product ID since it already exists. If you’re adding a product that’s new to Amazon, you may need to purchase a UPC code or request an exemption.
Where do I check if my GTIN is accepted? > Check here: https://gepir.gs1.org/index.php
How can I get an exception? If your products do not have GTIN, we advise you to get your products equipped with GTIN to avoid the exemption process. Nevertheless, you might be eligible to request a GTIN exemption for some of the scenarios listed below. First, check the Amazon catalog to see if your product already exists. If your product matches an existing product, you can add your offer on the existing product detail page without a GTIN. However, if your product does not match an existing product, you need to request a GTIN exemption and then add your product.
Examples for GTIN exemptions if you want to sell:
• Products for which the brand, manufacturer, or publisher does not provide a GTIN. For example, private-label products or handmade products.
• Products for which you are the manufacturer, brand, or publisher and you do not have barcodes on your products. For example, private-label products or handmade products.
• Product parts that do not have a GTIN. For example, automotive parts or mobile accessories.
• Bundled pack of more than one product. For example, a pack containing a leather belt and wallet or a pack of two shirts.
Detailed step-by-step guide how to request a GTIN exemption can be found here
However, some brands require a GTIN, check the list here