I believe you have to remove the requirement for a password. Roon Core has no way to provide that password.
I had the same issue when I first bought a HomePod and Roon support diagnosed the problem. I’ve been happily playing to my HomePod ever since.
P.S.: Roon can’t play to paired HomePod because that requires AirPlay 2. But it can play to a single HomePod as long as you allow streaming without a password.
From my Home app, I select Home Settings…
!
