Do you get this error in the Oculus Browser with Oculus Quest working as a stand-alone VR headset? Or in your desktop browser with Oculus Quest connected through Oculus (Air) Link?
Web VR experiences require HTTPS connection. Open the scene in using the HTTPS (https://) protocol instead of HTTP (http://) and the VR mode will be available.
Also, you may configure your own hosting to automatically redirect from http:// to https:// URL for the scene.