Hello @Fazal_Ashfaq welcome to Zwift forums.
As Gerrie noted, most of Garmin’s heart rate monitors broadcast only in ANT+. This is a problem for anyone running the Zwift game on a device that only “speaks” Bluetoooth natively. Most OS’s only support Bluetooth natively, and require an add-on ANT+ dongle to see Garmin’s HRM’s.
Fortunately for you, many Android devices made in recent years do support ANT+ natively. There may be steps you have to take to fully enable the ANT+ radio. Those steps are covered in our Android game FAQ. Please make sure to read through the section called Recap of ANT+ requirements: