Whichever method you choose, when pairing via the app, a tick on the LED screen means you have successfully paired. You will then need to reset the micro:bit and re-enter pairing mode to flash the code.
Another way to check Bluetooth is working is to use the NRF connect app to see which services are running. To do this, write a program with some bluetooth services enabled 'on start' and set to No Pairing Required then download to the micro:bit and connect to it with NRF connect. You should see the services that you enabledin the program, visible in the app.
Keywords for search: tick, pairing, ble, bluetooth, won't pair, won't flash, pass code, PIN, pattern