Check that the SteamVR Room View works. The layer uses the SteamVR camara APIs, so if that isn’t set up right, it won’t work.
You can also look at the logs in the debug tab to see if there are any errors printed. The log also gets written to C:\Users\<User>\AppData\Local\XR_APILAYER_NOVENDOR_steamvr_passthrough.log