So I'm reading the Unity documentation on Unity.XR.

It says, to convert a project to Unity.XR, use X menu item.

I use X menu item. It prints an error saying:

"You have more than one camera in your scene. We are unable to automatically convert your scene. Please see the documentation on how to upgrade your scene."

That's what it says. No indication which documentation or where. Just "The Documentation". The documentation says to use a menu item which says to read "the documentation".