Android system will report a “Couldn’t pair with (your device name) because of an incorrect PIN or passkey”, if the system cannot automatically pair with your new Bluetooth device, when RaceChrono connects to it when starting a new session for the first time.
Your device may have a button that needs pressed to allow pairing (for example on OBDLink LX and MX). Press that button and your device should now connect. You need to do this only once, after which the device is paired, and will connect automatically the next time.
If that does not work for you, you can also go to Android’s Bluetooth settings an pair the device manually, before starting a new session with RaceChrono. The device may request a PIN code that you have received from the device manufacturer, or just a default PIN such as 0000 or 1234.