Check the following:
Bluetooth is enabled on your smartphone.
Your device is powered on and not connected to another phone.
The app has the required permissions.
The device is within range.
Additional steps:
Restart the app.
Turn the device off and on again.
Remove any saved instances of the device in the app and attempt pairing again.
If pairing still fails, contact support at support@bookoocoffee.com with your device model, smartphone type, and screenshots or videos if possible.