You may see this message when using the latest version of the iOS app.

This is an issue with the software installed on the micro:bit. 

1: Program ("Flash") this hex file (attached) onto your micro:bit via USB (instructions here) to get the latest version of the micro:bit pairing software. Note that this should be done in normal operation and *not in maintenance mode

2: Before any kind of Bluetooth communication with the phone or tablet (pairing or flashing), enter pairing mode

3: Pair your micro:bit with the app using 'Choose micro:bit'

*If you try flashing this in maintenance mode a FAIL.txt file will be created on the device with the message:

The hex file you dropped isn't compatible with this mode or device. Are you in MAINTENANCE mode? See HELP FAQ.HTM