For some reason, I can't go to the "Blitz Moors" dungeon at Kania Wastelands since there's no select unit boxes under the deploy button. I'm thinking it is because I didn't advance the plot, but I finished all the current event that are available unless there's one I'm missing. Any help is appreciated. Thanks.

If you're talking about the "Last" one, you should be able to choose Deploy even without selecting any units.

Just deploy, and then manually choose who to bring out once you're in the map.