I see a blank screen

When this problem occurs during the video or photo steps, it is typically due to webcam utility software, virtual camera software, or security software that is installed on the computer. Examples of webcam utilities include Cyberlink YouCam, OBS Virtualcam, Droidcam, Webcam Splitters, ArcSoft Webcam Sharing Manager, or utilities labeled as Virtual Cameras.
The most common is Cyberlink YouCam, which shows up as Cyberlink Webcam Splitter in the pulldown list of webcams.

These webcam utilities intentionally manipulate the camera's video and/or picture modes so that they can apply effects to those modes.

Virtual cameras and virtual camera software are not supported with LockDown Browser. You will need to use a physical webcam that does not depend on virtual camera software in any way.

In many cases, students need to uninstall the software entirely for their webcam and browser to work properly. To do this, navigate to the Windows Control Panel > Programs > Uninstall a Program and select the appropriate webcam utility. On a Mac, students can use the Activity Monitor to force quit the utility software by going to Applications > Utilities > Activity Monitor > Select the utility app > Click the Stop button > Choose Quit.

Security software (anti-virus) can also cause the problem. You should look at your anti-virus settings to see if there are any camera specific or "real time protection" settings you can toggle off. You should re-enable them after your exam is submitted.