Hi Scott,
I was recently playing Pokemon TCG with my brother. He had a Sabrina's
Ghastly as his active pokemon (I foget what I had as my active pokemon). His
Only attack reads: SPOOK The Defending Pokemon can't retreat during your
opponent's next turn. 20
Does that mean that I can't use a switch?

Please Answer,
Tristan

No.  Retreating is defined as the act of paying the retreat cost and the Pokemon going to the bench because of it.  Switch circumnavigates that rule but allowing the active and a benched to "switch" places.  Nothing that triggers on or because of retreat triggers on switch.

Scott