WARNING: product is missing a field: google_product_category
You may be wondering what Google Categories have to do with Facebook anyway, after all, you are advertising on Facebook and not Google.
The answer is Facebook uses Google taxonomy to categorize items. If you do not know what that means, it's fine too. Just know that when the google_product_catagory field is not mapped, it will give you the warning. However, this warning can be safely ignored and it will not affect the performance of your Facebook ads.
If you want to use and set the google_product_category, you can use shopify's Google Shopping app to do so.