See the additional details in this Sonos support page…

https://support.sonos.com/en-gb/article/stream-airplay-audio-to-sonos

Some further info…

I’ve seen a few occasions reported, where a user has entered an AirPlay password for another iOS ‘Home’ on their old (no longer used) iOS device and that earlier Home was involved when their Sonos product(s) were first setup as an accessory in the HomeKit App. So check any older iOS devices too (if still available), or check the ‘Home’ of any others that are using HomeKit on the LAN.

Note this may also include Apple TV device ‘HomeKit settings too.

I would also try a reboot of the local router and power-cycle all your Sonos products.

If you can’t find the password protected Home, or it’s been deleted and you’re unable to resolve the issue, then the easiest answer maybe to factory reset the Amp and set it up again and then add it to your current HomeKit App as an ‘accessory’, with no password in situ.

Important Note: if you FR the Amp and it’s your only Sonos device, you will wipeout your Sonos Household and will lose all settings/services/playlists etc. so it’s entirely at your own risk if you decide to do that (as a last resort).

Your iPhone has an MDM installed, likely from your employer. This will enforce PINs on all AirPlay connections, but Sonos do not support PINs (because obviously they don’t have a screen to display it).

Sadly this common problem is missing from the support link above ( @Corry P could you get that fixed please?)

If you cannot disable the MDM, you will have to AirPlay from a different device.

Hi, thank you for your reply. My iPhone is unlocked and not supplied by my employer.

I even had a support call with Apple and that didn’t help. 

However, I have just factory reset the Amp and voilà. It doesn’t ask for a password and connects the iPhone to the Amp. 

SOLVED!

I can also add it to the Home app without it asking for a password.