The challenge requires you to have 24 skeletons out at once. That was only possible with the old useless Army of the Dead where the spell didn’t unsummon all your current skeletons to bring out new ones. But with the rework to Army of the Dead last patch, the talent now unsummons all your skeletons to give you a fresh new 12 skeletons, in short, you will never ever get 24 skeletons.
So…you made a useless talent even more useless. Normally with a change like this I suggest adjusting the Challenge. But since Army of the Dead is useless right now compared to the two objectively superior talent, could you just turn it into something more useful instead (while also making that challenge achievable!)