Certain versions of Magento may not come with native barcode support. You can either use a plugin that can add a property named "upc" to your products, or you can create a non-configurable attribute with the attribute code "upc" which will fill in for the missing upc field on listings. From there, all new listings imported from your Magento store will come in with the barcode you've entered in your store when creating the product.

Note: As with many channels the barcode field is named "upc" but it does not have to be an official Universal Product Code. You can use text, or any number you wish in this field.