You should have a look at your barracks. I am quite sure that you converted one of your storages, because you were supposed to have four at your level. You have to reconvert it (and then choose another building zu convert it into a barracks, I would recommend a forge).
The Devs should treat this as a bug. So many players have this problem, 4 weeks ago, I was in the same situation and after days without any progress, I wanted to delete the game.
Pretty sure, a lot of players quit, because of this problem!
I don’t agree. I think it is a really good idea to convert one building of your own choice. Which one is the best depends on at which stage of the game you are and how you play. With a stronghold at level 20 you don’t need that many storages, but maybe you want to produce a lot of items for fights within a short period of time, so you need another forge; some players even convert the houses, as this is the only building that makes sense for them.
Someone who brought his stronghold to level 10 should already know how the game works and should be able to deal with the consequences of every action within the game. So I am quite sure that it is only a really small percentage that does not remember which building was converted into a barracks.