I'm doing the mission called Old Guns. I'm at the part when you have to build and assign artillery at the castle. I already put 2 artillery pieces in the castle but I still can't progress. I tried talking to Ronnie Shaw after and she says, "But what are you waiting for? Get that artillery settled". What is it that I didn't do or did wrong? When I went back in the workshop menu, it says that I have to use artillery smoke grenades to call in artillery strikes on your enemies. I have no idea what that means. Please help. Thanks.

Did you assign someone to the artillery?

Have you assigned a settler to the artillery?

Also it's wise to put the artillery where the old artillery was (scrap the old ones)

Yep, like Ikill said, you must assign one of the people to use the gun. Then you'll get a message to go into town and toss a smoke grenade, mission done.