If you have multiple USB cameras connected to your computer (e.g. an external webcam and a document camera) and are encountering a "Permission denied" or similar error when trying to switch between them, this is likely due to usage of a USB hub.
If you are using a USB hub and are encountering this issue, try connecting only one camera through the hub and the remainder directly to your computer.
Cameras tend to use a lot of data bandwidth and having several of them active on the same USB hub can exceed the hub or USB port's limits.
This issue is common when a document camera is involved, as their higher resolution utilizes a significant amount of data bandwidth.