It actually needed to be put in a separate line and under a function. The Start or Awake functions get recommended for random range. But with those functions, the random interval seemed fixed between attacks and only changed when a new game started. However the Update function works for changing it between attacks.
I didn't think it would take that much research and be that complicated just to replace a fixed number with a random range. But at least I was able to do it. I should be able to get quite a bit more fun out of this game now.