UPC/EAN is required when adding or updating Items on Amazon

In most cases, Amazon requires providing a valid barcode when you List or Revise a product with a certain Product Type assigned.  

Since you are about to create a new ASIN or update your product details on Amazon, the Product Identifiers settings for Amazon Integration must be configured. Please navigate to Amazon > Configuration > Settings > Main > Product Identifiers and make sure the right Magento attribute containing a valid barcode is set for UPC/EAN field.
 

You can also set product IDs per M2E Pro Listing during listing creation or under Listings > Items > Edit Settings > Selling > Product Identifiers. Please note that Product ID settings adjusted at the Listing level will override any previously configured global settings.

For listings with variations, Amazon expects a unique product identifier, such as EAN or UPC, to be provided for each variation. For example, if you are selling a T-shirt in different colors and sizes, each variation would require its own unique barcode. So please confirm that all of the Child Products in Magento contain the attribute with a valid UPC/EAN code. No barcode is required for a Parent Product.  

You will be able to List and Revise your products on Amazon without a barcode only if you have been approved for GTIN Exemption. In this case, the Product Identifiers settings are not required, you can find additional details here.