Sounds like a pairing problem with the phone and needs to be redone. Make sure to use the Garmin Connect App (not android Bluetooth settings) to pair with your Garmin with your phone. Follow these steps and video:
Remove all your Garmin devices from from Garmin Connect App (Menu >> Garmin devices)
"Unpair" all Garmin devices in android Bluetooth settings
Do the same for any other nearby mobile devices that might have Garmin in BT settings.
Reboot your phone, restart the watch
On your Garmin device place into "pairing mode." Differs per device, but usually in Settings > Phone > Pair Phone to manually enter pairing mode.
Pair using Garmin Connect Mobile app
If after this, you're still unable to pair, try clearing the Bluetooth cache as follows:
Open the Settings app
Tap Apps.
Tap the menu icon and then choose Show system apps.
