After selecting the area to record, the area inside of the selection is just white.
This is a known issue that will be fixed in a future update for Snagit. This behavior can occur when the web camera is enabled, and the initial area that has been selected is re-sized.
Option 1: Restart the capture
This problem can be resolved by simply restarting the capture using this button:
Option 2: Turn off the web camera
This can also be avoided by turning off web camera recording in the Snagit capture window.