There may be a number of reasons why Amazon has not accepted your GTINS/ barcodes.
1. Invalid error: usually due to not selecting EAN as the product ID type

A common problem is selecting the incorrect product ID for a listing.
You can resolve the problem by following these instructions:
When you upload your product into Amazon Seller Central, the first attribute you are asked for is the product ID. This the unique GTIN provided to you by GS1 Australia (if you are selling someone else's products then you will need to ask them for numbers).
Amazon allows you to add:
- UPC - Amazon is expecting a 12 digit number. Amazon is expecting a 12 digit number. It is unlikely that you would select UPC as the 12 digit number, as these are for US retailers only.
- EAN - Amazon is expecting a 13 digit number. Therefore, if you are selling a single product, sometimes called a SKU or a child product if you have variants, you will need to select this option.
- GTIN - Amazon is expecting a 14 digit number. If you are selling a case or box of products - for example, a carton of 6 products (that would usually be sold separately) all together, then you need to select GTIN and input the GTIN-14 that you have assigned.
- GCID – is a number that Amazon assigns to your product, if you have a GTIN exemption.
Note: After April 17, 2018, GCIDs can no longer be used in place of a UPC, EAN, JAN or ISBN to create ASINs. If your brand has been approved for the new brand Registry programme and you do not have UPCs, EANs, JANs or ISBNs for your products, please apply for GTIN exemption.
2. Barcodes do not match
This is usually an issue with the brand name being different to the company details registered with either Amazon or GS1 Australia. When you receive an error message, you will need to provide Amazon with proof of the connection between the seller and the brand. Examples of proof include trademark certificate, invoices to show that you purchased the stock from the brand or a letter of authorisation to prove that you have permission to sell. For more information, contact Amazon Seller Support.
3. Potential match
You will receive a message stating “We found possible matching product(s)." This means that the product you have listed may already be in the Amazon Catalogue. You will need to check if it already exists.