I can't sign in Zepp app, what should I do?

Check if you enter correct user account and password.
Switch the network from WIFI to 3G/4G in the settings of your phone.
Clear Bluetooth Share data and erase app cache/data
Reboot your phone
If the above steps don't work, please uninstall the app and then re-install it from the Google Play Store/App Store to have a try. If the issue persists please submit feedback via Zepp app, write an email to describe your problem and send your ID to the customer support.