Welcome to the Forums @SummerMarcelliu.
This might be happening because the Xiaomi Mi5s is not part of the supported devices list. This means that the app has not been fully optimized to work with that phone.
As long as it fulfills the minimum requirements, Android 4.4 and Bluetooth 4.0, there is a possibility that it can work but we cannot guarantee it. Some users of unsupported devices have mentioned that following these steps helps with the syncing.
If none of the above works, you could try to use a computer or another mobile device to try and sync your Fitbit to your account. Click here to get instructions on how to sync with other devices.
Let me know if you have any further questions.
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