What do you mean with slot B?
I mean, my guess is the second slot for spells in act 3? You can’t get it yet, not sure when you’ll get it but it’s not now.
If it’s not that, please clarify.
Normally you have to talk to her, she introduces you to the war room and then you can train with her to start conquering.
If you somehow got into the war room before being initially introduced to it, the game accidentally skipped something, but it’s nothing major, It’ll be fixed on the coming updates.