Elle here from Shopify.
If 1 product is not taggable, it may be due to an error syncing to the Facebook Sales Channel. Double check if it's synced to Facebook first and foremost. If it isn't, head to the product and ensure it's set to be available on Facebook and Instagram. Also make sure it's set to require shipping and has a product description.
If the product is synced to Facebook already, then we'll need to take a closer look. Let me know either way!
Hope this helps. :)