I’ll be sending my Watch 6 back for a full refund if this is not fixed by the end of next week. That’s when my 14 day return ends.

I have the same issue. Spent a long time on the phone with Apple. They said it had to do with the beta iOS 14.2. I deleted the beta And now I am on 14.0 (18A373) with OS7 beta on my watch 6. Still does not work, very disappointing. The blood oxygen app does not appear on my watch or in the App Store on my watch.