Apple Watch SPO2 readings show zero in the Health app Vitals

Not all Apple Watches are designed to provide SPO2 data.

 The Blood Oxygen app is not supported on Apple Watch SE or Apple Watch for Your Kids. For models of Apple Watch purchased in the United States on or after 18 January 2024 with part numbers ending in LW/A, the Blood Oxygen data analysis is performed on iPhone, and results can be viewed in the Health app. This experience is available on the latest version of iOS and watchOS. Find out how to identify your Apple Watch.

This isn't the first post I've seen to report this. And, my experience is the same as yours. I submitted feedback.