I suggest adding an administration setting (in the Whiteboard panel) to make this error message disappear from the "Overview" admin panel:
"WebSocket server URL is not configured. Real-time collaboration requires a separate WebSocket server. Basic whiteboard functionality works without it."
Please refer to the Nextcloud Talk case when HPB is not configured; an option in the Talk administration settings allows you to hide the warning and ensure that the instance verification turns green.
Nextcloud Talk mentioned setting :

Where you should add the same kind of option to hide the Whiteboard error message in the "Overview" panel :

The Whiteboard error message I'm talking about :

I am making this suggestion because you are not preventing people from using Whiteboard, and it is simply necessary for them to know that online collaboration is not possible without Websocket, just as it is possible to use Talk without HTB.
The idea is for administrators to be aware of this, without scaring them.