Just talked to Ronnie Shaw at the Castle. Followed her around after having an awkward conversation with her about why she is still in the minute men. Now she wants me to blow up a rubble pile that is blocking the way into the armory. My HUD is telling me I have to do something at the work bench. I can't seem to figure out what I need to build I suppose. What could I possibly have to build to move rubble? I missing something?

You have to harvest the stone by scrapping it.