Garmin devices connect via ANT+. Just about every other piece of technology connects wirelessly via Bluetooth instead. The two technologies are not compatible. Recently, Garmin have started to produce heart rate straps with dual ANT+ and bluetooth, so this situation should improve in the future.
If you don't have a Garmin heart rate strap with bluetooth in it, It's possible to use a bridging device such as the North Pole engineering CALBE, or the Viiiia Heart Rate Strap. These devices convert ANT+ to Bluetooth.
Alternatively, if you have a Wahoo KICKR or another trainer that sends power but not cadence, the Wahoo TICKR X is an excellent choice.