I suggest you to try the following steps:
1. go to sync option.
2. press 'play with phone' .
3. go to sync option and press 'Mi band' to reauthorize your Mi account.
4. Back to Wokamon home page and try sync with Xiaomi again.
If you tried the aboved methods and still can't fix the problem, you can try:
- go to the settings.
- Press 'save game'.
- Uninstall wokamon then reinstall and log in the same account to see if it will successfully sync with Mi band.