Failed to access your webcam
If you are using the "embed webcam" feature or "record webcam", Screencastify might fail to start recording if cannot capture video data from your webcam.
Re-starting your computer will most likely resolve such issues. If you have an external webcam, it might also suffice to just re-plug it.
You might see this happening if you have recently installed system or driver updates and did not restart your computer since. Usually video issues are not tied to Screencastify in specific, but affect other applications as well.