There is a known issue with the virtual camera feature in recent versions of OBS.
What happens is that OBS will give an error message that says it can't find the virtual camera, when you try to enable it.
To fix this, you just need to log out from your account and then log back in again.
this will trigger macOS to check again for things such as the virtual camera. Now it will work again.
From my testing, this only appears to need to be done once after updating OBS.
Hope this helps.