Failed to load NaCl module
You usually see this error when Chrome fails to run NaCl Applications. To verify if this is an issue with Screencastify in particular or NaCl in general, you can try if this Demo (which also uses NaCl) works.
The first thing you should try in this case is to see if it works after restarting your computer.
If this does not fix the issue, try if it works after re-installing Chrome. In this case, make sure to backup any recordings that you did not store on Google Drive first. As re-installing Chrome or Screencastify will delete all locally stored recordings.
If the demo works, but Screencastify still does not, please submit a bug report. Then something else is off.