I use Polar H10 heart rate monitor, is it compatible with the app Garmin-Connect installed on my iPhone X Max?
True. Just need a Garmin fitness device along with the h10 to get it on Garmin Connect. Sounded like the OP was asking about using the h10 by itself.
You have to connect your polar h10 with a Garmin device, then you can sync with Garmin Connect.
I have no problem with my Polar H10 and Zgarmin fenix 5 plus. They syncronise.