First check the product packaging, if there are no barcodes, then remove brand and MPN values and set identifier exists to no. Even if you set it up correctly in the free google shopping app, it’s bugged and does not correctly send data to Google.
Therefor, you need to remove the brand and mpn using feed rules, and set identifier exists to no.
Here is the official guide: https://support.google.com/merchants/answer/7450276?hl=en
Then after that reprocess the feed so that all products read the new rules. You can do this by editing all your products in shopify, or add a supplement feed with just the ids to refresh the rules.
Here is a full guide: https://www.youtube.com/watch?v=AyvilyMb2Bo