DIAGNOSIS The pairing handshake between your phone and the grill's module is failing. This is more often an app/phone issue than a grill hardware issue.
FIX The nuclear option that usually works: Delete the Pit Boss app from your phone. Power cycle your phone. Reinstall the app. On the grill, with it plugged in but *off*, hold the Bluetooth button on the control panel for 10 seconds until the light starts flashing rapidly (this resets the module). Then try pairing fresh in the app.
IF THAT DIDN'T WORK Make sure your phone's Bluetooth is actually on and you've granted the app all permissions (especially location, which Android often needs for Bluetooth). Try with a different phone to isolate the problem.
HEADS UP The Bluetooth range is terrible, like 30 feet *max* with clear line of sight. And the connection drops if you walk away. It's really only for monitoring from the kitchen, not true remote control from the store.