Whatsapp web - Camera unavailable

Good afternoon,

I am trying to use Manycam with Whatsapp web so that I can video call with a prerecorded video. However when I start a video call and change to the Manycam virtual camera on the app I get the error message “Your camera is unavailable or in-use on another app”.

I cannot find a workaround for this. Can anyone assist me please?

Thank you!