Try going through each one, one of them might actually be supplying webcam data. I don't know what this bug is, it pops up every so often.
Things you can try: running as admin, restarting your pc, whitelisting vtuber kit in your antivirus, plugging your webcam into a different usb port, removing other peripherals when booting vtuber kit.
Eventually this issue clears itself up, but I have no idea why or how. Hopefully one of those things works for you