First, confirm you have Bluetooth turned on, on the smartphone.

Confirm that the device is powered on and is responsive to button presses. 

The app scans for devices periodically, so user can try waiting for a minute or so for initial connection.  Typically device is seen in 5-10 seconds but there may be differences based on the OS/ version / hardware. 

If the device does not connect within a minute, turn the device off or unplug it, and close the app. Then, turn the device back on to refresh the Bluetooth, and reopen the app.  

In rare cases where this does not resolve the issue, please delete the device from the app vehicle.  Unplug the device. Close the app.  Go to your phone's Bluetooth menu and if the device is listed, remove/forget it. Restart the phone.  Then plug the device back in, open the app, and add it back to a vehicle in the app. This should resolve the issue.