This post is a part of a larger thread that includes current known issues & required permissions to successfully connect to the Facebook Channel. You can find the Table of Contents for all required permissionshere.
When clicking "check out on website" on a product page in your Facebook shop, the customer sees "required parameter missing or invalid: channel message" and can't proceed.
This error is likely due to the settings not switching over correctly when migrating from "Facebook Shops" to the "Facebook Channel." To fix this, you will need to disable "Use Checkout URLs" in your Facebook page settings under “Shop tab:”
Once you have turned this off, please make the affected products unavailable to Facebook in your Shopify admin, then make them available again. More information on how to change the availability of a product can be found here.
If you are still having issues after you have disabled "Use Checkout Urls" then please reach out to Shopify Support for further assistance.
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog