@ Jenei Complete the questline for the Battle for Lordaeron scenario and you should be able to start the Allied Races questline. From what I can tell you have indeed completed the requirements!
09/08/2018 08:55Posted by Sorelay
... since I have noticed, even though the achievement is accountwide open ... if the Horde character hasn't completed the requirements .. it isn't unlocked ! :'(
This is a bit inaccurate. You only need to unlock the Allied Races on one character-faction. If you have another 110 character level on the opposing faction, they should be able to start the questlines to recruited Allied Races for their faction.