The issue can either lie with the input device (webcam, capture card, etc) or with the MixCast configuration for the device:

First, ensure that your Input Device is connected and accessible to the system:

• If this is the first time your camera/capture card has been plugged into your computer, restart the computer to ensure all changes (driver installation/updates) have been applied.
• Check if any other programs are accessing the input device already.

Next, in MixCast configuration:

• If your camera/input device has multiple possible resolutions, frame-rates, or pixel formats, try different combinations of options in case specific settings are incompatible.