Failed to capture audio
In some cases Screencastify might fail to start recording if cannot capture audio from your microphone.
Re-starting your computer will most likely resolve such issues. If you have an external microphone, 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 such audio issues are not tied to Screencastify in specific, but affect other applications as well.