To resolve the issue of not being able to change the watch face on your F Fit device and receiving an error message, please follow these steps:

1. Restart Your Devices: Begin by restarting both your F Fit watch and the associated mobile app. A fresh start can clear temporary glitches that may be causing the issue.

2. Clear the App Cache: For Android users, go to your device’s Settings > Apps > F Fit app > Storage, and then select "Clear Cache." This action can help eliminate any corrupted data.

3. Update the App: Ensure that the F Fit app on your mobile device is updated to the latest version. Go to the Google Play Store or Apple App Store and check for updates.

4. Reinstall the App: If clearing the cache does not work, consider uninstalling and then reinstalling the F Fit app. This can resolve deeper issues that might be affecting your ability to change the watch face.

5. Check Permissions: Make sure that the F Fit app has the necessary permissions enabled on your mobile device to function correctly. You can check this in your device's app settings.

6. Verify Bluetooth Connection: Ensure that your watch is properly connected to your mobile device via Bluetooth. Sometimes, a weak or unstable connection can cause difficulties in changing settings.

7. Check Watch Battery Level: Make sure your watch has sufficient battery life (ideally above 30%) as a low battery can hinder app performance.

8. Factory Reset (if necessary): As a last resort, if the problem persists, consider performing a factory reset on your watch. Remember that this will erase all data, so back up any important information first.

If after these steps you are still unable to change the watch face and continue to see an error message, consulting the manufacturer's support team may provide specific guidance tailored to your device.

For additional help with similar issues, you can refer to the Garmin forums learn more or check for updates and common solutions there as well.