I reached lvl 21 on my mage and the second slot that opens at level 21 is not unlocking. I tried restarting and no luck! Looks like a bug? Any help with this is greatly appreciated.
It’s actually based on item level right now, so you need to find a level 21 weapon or above.
Also, this is going to be changing for the next patch, unlocking the skills at character level 10 and 20 instead.