The Garmin Edge 500 is unable to finish calibrating or may be very slow to calibrate if it is still searching for other non-paired sensors. This will happen if there are sensors enabled that are not present (or no longer being used) or they have not been activated/paired, i.e. your speed sensor has not been activated by spinning the wheel or your HR strap is not being worn at the time.
Steps to Complete Calibration
First option: Pair Sensors
To pair the sensors, simply wear the HR strap until it is detected by your Garmin and spin the wheel and/or crank until the magnet passes the sensor to activate the Speed/cadence sensor.
Second option: Disable Sensors
To disable the HR:
Settings > Bike Settings > Heart Rate > ANT+ HR > and change the selection to "No"
To disable the Speed/Cadence:
Settings > Bike Settings > Bike 1 (or applicable bike profile) > ANT+ Spd/Cad > and change the selection "No"
Still need help?
If this does not fix the problem, please open up a support ticket with Stages Cycling or visit your nearest Stages Cycling dealer for assistance.