Hi Emily,
Thanks for reaching out! We understand that you're no longer able to access the Blood Oxygen app from your Apple Watch Series 6, and we're committed to finding a resolution. To clarify, are you still located in the same region where the Apple Watch was originally purchased? The reason we ask is that the app may not be available in all regions. If you are still in the same region, let's work through the details in the resource below, and see if we can isolate the problem a little more.
How to use the Blood Oxygen app on Apple Watch Series 6
Please note that there are a few instances where the Blood Oxygen app might not be available including Apple Watch set up using Family Setup, certain regions and users with an Apple ID profile under a certain age. If you continue to run into problems, please be sure to let us know which step you get stuck on, and we'll keep working from there.
All the best.