What type of trainer are you using? When in the pairing screen are you choosing speed to search for your Garmin sensor 2? Make sure nothing else is trying to grab that signal like your phone or a bike computer.
You may have revealed the issue. Bluetooth can only connect to one device so if your trainer is pairing to you your watch then Zwift can’t grab it. Make sure your BT on your watch is off or if there is a way then unpair it while you’re Zwifting.