The AirPlay device “x” cannot be selected for audio output. Please verify your network settings and AirPlay configuration or select another device.

FIXED: (THIS IS A FYI for humans who, like me, couldnt find the solution.)

I was faced with this today. Using Music app... been using like this for years without change... and a speaker that used to operate from my iMac last week no longer works, but in addition to not working - I was offered this text. Googled it. Found an old thread that I could not reply to.. but I found the latest solution. The built in Apple FIREWALL! It [must have] been automatically activated after upgrading to Sonoma 14.1 (23B74). I have not tinkered with my firewall for years, so it must have been enabled during the upgrade process to the point where NO NEW INCOMING CONNECTIONS was enabled... so all the App rules I created were overruled by this.

I got lucky and saw this pop up on my secondary monitor which is facing away from me.