DIAGNOSIS Yeah, this one's common. The Beam is stuck in a setup loop, usually because its network configuration got scrambled or it's trying to connect to a 5GHz-only Wi-Fi network it doesn't like.
FIX You need to force a factory reset. Unplug the soundbar. Hold down the Play/Pause button on the top of the Beam (the one with the three dots) and, while holding it, plug the power back in. Keep holding until the light on the front cycles through orange and then goes solid white—about 10 seconds. This wipes its network brain. Then set it up fresh in the Sonos app as if it were new.
IF THAT DIDN'T WORK If it's still stubborn, the issue might be your router. Temporarily disable the 5GHz band on your router (or create a separate 2.4GHz network) and try the setup again. The Beam Gen 2 can be picky during initial setup and needs a solid 2.4GHz connection to start.
HEADS UP After the reset, you'll have to re-add it to your Sonos system and re-run Trueplay tuning if you use it. All your music service logins are stored in the cloud, so you won't lose those.