If you have met the requirements for an Activity Challenge, but the achievement has not yet been awarded, then Apple advises that you should allow nine days for it to be awarded:
Earn Activity achievements using your Apple Watch - Apple Support
The following steps may also help:
If you have not yet done so, then update your iPhone and Apple Watch (not iwatch) to the latest software, starting with your iPhone:
Check your personal information:
- On your iPhone, in the Watch app, go to: My Watch (tab) > Health.
- Check all settings, including selecting Yes or No for Wheelchair use rather than Not Set.
Close certain apps and then restart (normally) both your iPhone and your Apple Watch:
Unpair and re-pair your Apple Watch:
If the issue then persists after waiting for nine days, then I suggest that you contact Apple Support for assistance:
(Note that you are not talking to Apple here - this is a user-based technical support community).