Why the browser ask me to access cam when i start the videochat?
This is normal and it's due to the privacy policy adopted by all browsers. Every browsers don't allow unauthorized access to the hardware until users agree.
Depending on specific browser the message shown to ask cam access will vary, as visible in screenshots below.
The videochat application of course can't start if the user doesn't grant access to webcam. Notice that in order to allow the browser using the pc webcam there shouldn't be any other applications using it. Access to hardware can occur only by one single application at a time.