It is a recuring bug lately to have Cannons not mountable on the Armitage, being introduced by patches. Then it takes 3 or 4 days to be fixed, only to come back next patch.
This is caused by some mixmatch in the following data:
The Cannons do not List Heavy Escort Carrier as a compatible ship type but do list Escort Carrier.
The Armitage is a Heavy Escort Carrier, which is supposed to be a subset of Escort Carrier.
There is only 1 Escort Carrier in the game right now and it is the Armitage, which is a Heavy Escort Carrier.
I would think that for a $20 pay to play ship, they would be more careful about this sort of stuff, but clearly they have decided the game needs a ship that can be considered a Lemon and right now the Armitage is it.
UPDATE: Forked out some chump change in EC and tested it this time to make sure I'm right and this time it seems that it's just misreporting on the dual cannons, but does still work.. I tested this when I realized it is saying that hangars can't be mounted on the Armitage either... Last time this bug was around it wasn't just cosmetic, though... I ran into it when it wouldn't let me mount a new dual heavy cannon a couple weeks ago.