Merchants that are using Checkout on Facebook or Instagram are required to have Google Product Categories (GPCs) for all products available on Facebook. If you're using Checkout on Shopify, you aren't required to have GPCs. Adding them regardless can be a good idea as GPCs can increase the discoverability of your products on Facebook and Instagram.
If you're receiving an error that your products are missing the Google Product Category, you can navigate to the Facebook sales channel within your Shopify admin and click on the Not approved products in the Product status card to be taken to the bulk editor, and there you'll be able to fill in any missing GPCs.
I hope this helps! Please let me know if you have any other questions.
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