Unable to max Magic Guard at level 40 and thereafter, if it wasn't maxed previously.

Wait why do you have 3 SP? If Lightning Bolt and Magic Guard both max at 20, then what are you going to do with the last point?

That's exactly why you can't put it into anything. Nexon made it so that the Cash Shop book gives you 1 SP, which gives you 1 more than you should have.

I wonder if skipping one of the SP quests will let you max it.