In order to use an Apple Watch with Natural Cycles, your watch must have a temperature sensor. This includes Apple Watch Series 8 and later, any model of Apple Watch Ultra, and Apple Watch SE3.
Please note that the Apple Watch SE & SE (2nd generation) models do not have temperature sensors, so they can’t be used with Natural Cycles.
If you have an Apple Watch but are not sure which model you have, this article may be helpful: Identify your Apple Watch
Learn more about how the Apple Watch works with Natural Cycles here: How does Apple Watch work with Natural Cycles?