On the Procaster launch screen just below the 'Go Live' button there is a drop down menu which allows you to select your input source. One of these options is 'No Camera'. When you go live with this option, you will automatically begin to capture your main screen. If you go live with a camera as your source, you can switch to screen capture by clicking the 'Screen' button on the Procaster HUD at the bottom of your screen.
If this does not solve your issue, please include a screenshot of the error message.