Video conference/call in EGroupware and Rocket.Chat are two completely different things!
In both systems, the video conference systems are configured respectively.
In both systems you can configure Jitsi or BigBlueButton.
Basically, you have to allow the browser to access the camera. The same applies to the microphone.
That is also a good thing. Otherwise, any website could switch on both. We certainly don’t want that.
If you have not allowed access in the browser, Jitsi/BBB will not be able to access it either, because they will not “see” any camera. The icon is then greyed out.
Another reason could be that your operating system has “given” the camera to another programme. You cannot access it from two programmes. It usually helps to unplug the camera and plug it in again.
This is a fundamental problem of any videoconferencing solution and has nothing to do with EGroupware.
Without naming the operating system / the web browser used and showing exactly what is happening with the called party (also the access rights in the browser), it will not be possible to say more about this.