For three weeks now (End of November 2021), Shopify merchants that use the free Google shopping app have started complaining that barcodes are no longer getting submitted to Google Merchant Center. And you will start seeing warnings such as GTIN is required for this product or Limited performance missing product identifiers. While you do have barcodes in your Shopify account.
Simply put, the issue is the free app you are using, which is no longer submitting barcodes.
I was expecting Shopify to fix the issue in 1 to 2 weeks, as I decided not to create a post offering solutions. However, it’s now three weeks, and still, nothing is fixed. Instead, you will get replies stating to contact Google support. And then Google will say contact Shopify.
In this case scenario, Shopify is incompetent in fully understanding the issue. So let’s not waste any more time and resolve the issue.
Solution 1 and 2, is currently not working, due to Shopify’s free app having a new bug, only 3 and 4 are now valid solutions, as of 26th April 2022
Go to Shopify > Sales Channels > Google > Manage Availability
All products with barcodes need to be marked as false under custom products.
As suggested by Dave in the comments, you can bulk edit by downloading all your products. Go to Shopify > Products > Export
Select all products and click on export products.
Open the CSV file that has been emailed to you and search for the column Google Shopping / Custom Product, and add the values FALSE to all products.
Save your CSV file and go to Shopify > Product > Import > check the box next to Overwrite any current products that have the same handle, and you are done.
If you don’t have many products, you can use a supplement feed to add the missing info. You do this by first downloading a copy of your data, then deleting all columns except ID, Brand, MPN, GTIN and Identifier Exists.
Edit your data so that all barcodes are added, then copy the data and create a Google Sheet. This sheet can then be added as a supplement feed, explained in the referenced article.
If you have too much data to edit, choose a different data feed app. For my clients, I use Multiple Google Shopping Feeds or DataFeedWatch, because it has all the features I need to fully optimize the data feed quality, which results in higher impressions and lower CPC. The link is an affiliate link. However, I would never recommend something I do not extensively use myself.