Open Apple menu
Click System Settings
Click Privacy & Security
Click Camera
Check whether your webcam app is allowed
Open Photo Booth
Open FaceTime
Check if the USB webcam appears as a camera source
Disconnect and reconnect the USB webcam
Try a different USB port
Open Finder
Click Applications
Open Utilities
Open System Information
Click USB
Look for the webcam in the USB device list
Open Terminal
Run `system_profiler SPCameraDataType`
Run `system_profiler SPUSBDataType`
Restart your Mac
Test the webcam in another app
Update macOS
Install any webcam driver or software from the manufacturer