Hello Brian.
A small mistake in the code according a spelling mistake.
In chapter 36 when you add ForceReceiver to Start in PlayerImpactState and EnemyImpactState, you writte:
ForceReciever.OnForceApplied += Handle_ForceApplied;
you invert the i and the e in ForceReciever.
The correct spelling must be :
ForceReceiver and not ForceReciever.
If not other student like me would be able to spend much time to understand what happen lol 
What is most need ForceReceiver than ForceReciever 
Have a nice day.
François