Diagnosis
The soundbar is stuck in a partial setup state from a previous failed attempt. It needs a hard reset to clear its network configuration.
How to Fix It
Perform a factory reset. With the soundbar on, press and hold the Power button and the Skip Forward button (the one with a dot and right-facing line) on the top panel for 10 seconds until the light bar flashes amber, then release. The soundbar will reboot to factory state.
If That Didn't Work
Ensure your phone's Bluetooth is on and you're connected to your home's 2.4GHz Wi-Fi network (not 5GHz) during setup. Delete and reinstall the Bose Music app, and restart your phone before trying again.
Heads Up
During setup, place your phone very close to the soundbar (within 3 feet). The initial connection uses Bluetooth Low Energy, and distance or interference can break it.