I saw this after I shared what I was seeing, so I did the same thing ECG then the Blood Oxygen app, it still had the message, I decided to wait a few more hours for the 24 hour period. I was telling my wife, and opened the app sitting in a park, it had taken two then the on-demand one. All's well with the world now.
"Apple Watch Blood Oxygen App Issues
Apple has reintroduced the Blood Oxygen feature for specific Apple Watch models in the United States via the watchOS 11.6.1 and iOS 18.6.1 software updates, but users are encountering issues where the app does not function as expected after the update.
The feature is now available on Apple Watch Series 9, Series 10, and Ultra 2 models sold in the U.S. that previously lacked it, but the functionality has been redesigned to process data on the paired iPhone rather than the watch itself.
Some users report the app remains unavailable or shows a "no longer available" message, which may be due to a delayed over-the-air asset download that can take up to 24 hours after the update.
The Blood Oxygen app is only available on Apple Watch Series 6 and later, including the Apple Watch SE (2022), Apple Watch Ultra, and Apple Watch Ultra 2.
A workaround reported by users involves opening the Heart Rate app and taking a manual reading, followed by using the ECG app to perform a full reading, which may force the system to recognize the Blood Oxygen feature again.
To potentially jump-start the necessary asset download, users are advised to open the Health app on their iPhone and the ECG app on their Apple Watch.
The feature's return follows a redesign to avoid infringing on a patent held by medical device company Masimo, which led to the original ban on new U.S. models.
The results of a Blood Oxygen measurement are now displayed in the Respiratory section of the Health app on the iPhone, not directly on the Apple Watch.
The update does not affect Apple Watch models sold outside the U.S. or older models that already had the feature."
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