This error occurs because you selected the UPC as an identifier, and the UPC code is a 12-digit code, but you are entering a 13-digit EAN code. As a result, instead of the UPC, you must now use the EAN as an identifier. If it is not editable from the backend of your product, you can choose the inventory file and update it using the inventory file. Alternatively, you can save your product’s data and delete it for nearly 24 hours, then relist your product with the same SKU and EAN number and choose EAN as an identifier. If this issue is still not resolved, contact Amazon and they will guide you on how to change this.