How to fix the connection error for cameras in Valve Index?
Aside from turning it on inside SteamVR menus, you may have to enable access to your camera through Windows 10 system settings menu:
To find this setting page just type "camera" after opening the Start menu and you should get a link to it. Flip the toggle button to On in the "Allow apps to access your camera" section. This should be enough. Because SteamVR is not a Microsoft Store app, it doesn't look like it will be listed in a list of per-app permission toggles below. Flipping just the one switch on top helped me turn on the cameras.