Fitbit does not support Apple Watch.
Some third-party apps enable data to be synced from the Health app to Fitbit. For example:
Sync Solver - Health to Fitbit on the App Store
However, Fitbit does not allow steps from third-party apps to be counted towards Fitbit challenges.
For assistance with the supported features of the Fitbit app, I suggest that you consult the developer's support resources and/or contact them:
Similarly, Apple Watch supports Activity Sharing with other Apple Watch users, but not with users of other fitness tracking devices:
Share your Activity with your Apple Watch - Apple Support
If you wish to use an Apple Watch and still participate in challenges with friends who use other devices, then you may find the following third-party app of interest:
- "Challenge your friends! You can directly challenge friends in activities such as step tracking and runs to bicycle distance challenges and more."
- "AC supports Apple Watch, Fitbit, Jawbone, Garmin, Misfit, Withings, Microsoft Band, Under Armour and more as well as fitness applications such as Apple Health App, Runkeeper, Strava and Google Fit."
- App: activity.club on the App Store