To mitigate against false readings from everyday activities, wrist temperature data is only recorded while you’re sleeping. This means you’ll need to wear your Apple Watch to bed every night with Sleep Focus enabled for at least two months before you’ll get a differential reading suitable, for example, for ovulation indication.
If you are trying to use it for that purpose please read Track your nightly wrist temperature with Apple Watch Series 8 – Apple Support (UK)
or each out to Apple Support directly. You should be able to reach them here
Click here > Contact - Official Apple Support
or phone numbers
Click here > https://support.apple.com/en-us/HT201232
and or
Genius Bar Reservation and Apple Support Options - Apple