I'm on this mission, as title says, and mistakenly used all the AP 203 mm mortars already loaded on the artillery. Next to the artillery gun, there is a box with 4 mortars of 50 kilos each. I cant pick them up, so, cant load the artillery gun to complete the mission. Why is that I cant pick up those things?

Your soldier has a weight carrying capacity, try dropping his secondary weapon or other stuff that might be weighing him down [ edit: Actually, it does auto-arrange the inventory if you have the ability to carry the new object. Ignore this =>] and make sure the items in his backpack are arranged so there is enough space to fit the shape of the object you're putting in (I don't think the game auto-arranges items to make them fit)

Also, extremely annoyingly the way that you actually load the artillery is by Dropping the shells onto the gun, rather than it having an inventory like vehicles, or even by Using them.

I've just spent an hour figuring it out and being on the edge of quitting altogether.

I'm starting to see why people like the game, but dear me are there a lot of rough edges that get in the way.