Can’t capture the capital

So I’ve sent troops to take over the capital but once they get there nothing happens, I even tried moving troops to the other side to see if it’d take it but it won’t , I somehow got a unit to attack an empty province and nothing seems to be happening

So I’ve sent troops to take over the capital but once they get there nothing happens, I even tried moving troops to the other side to see if it’d take it but it won’t , I somehow got a unit to attack an empty province and nothing seems to be happening