Catch requires retailers to supply the UPC/EAN/ISBN (collectively referred to as the GTIN, or commonly also known as the barcode) for all the products they want to list.
This field is a specific format and can't be "made up" by the retailer; it needs to be an officially assigned GTIN (see here for more details: https://www.gtin.info/). Depending on the source region of a product, it may have a UPC (Universal Product Code - common in the U.S.), an EAN (European Article Numer) or if it's a media product such as a book or a DVD, an ISBN (International Standard Book Number).
You can check GTIN existence here https://www.gs1.org/services/verified-by-gs1
To ensure you are providing the GTIN, you will need to assign it as follows in WooCommerce. If you have a custom product or you are selling own-brand items there may not be a UPC for that product then there are exemptions. Please check with Catch if you can still list based on its rules around which brands and categories of products are mandated to have a GTIN. Note: If you have your own brand products, add the brand name(s) under 'Settings - Catch - General - 'Own Brand'. This will bypass the requirement of having a GTIN.
A simple single-variant product has a single GTIN associated with it. Set it up as shown below, as a custom attribute of the product. The name of the attribute must be one of UPC, EAN or ISBN, as appropriate.
Each variant needs to have its own GTIN assigned to it. Each variant therefore needs its own value of the custom attribute, as shown below:
Then on the Variations tab you need to assign a GTIN to each variant: