Level 120s can do the achievement just fine. In fact, I finished it on my Warrior quite recently. I thought I had finished all classes long ago, but apparently forgot Warrior.
Since you’re obviously not posting on the relevant Warrior, I can’t check anything for you. Post on the character you’re having trouble with, or give us name and realm, so that we can look.
Don’t go searching through your class hall expecting someone to talk to to instantly complete this quest.
Champions of Legionfall can only be completed after you have completed the quest chain to recruit your new 7.2 follower.
The chain starts with a quest you get when returning to your class hall after accepting Champions of Legionfall.
Once you have your new follower, go back to Deliverance Point to turn in this quest.
It’s quite common to have started this quest and then just stop, later abandoning it somewhere down the chain. Have a look at the chain on Wowhead and either grab an addon or use this script to check what quests you’ve completed.
/script print(IsQuestFlaggedCompleted(QuestID)) where QuestID is taken from Wowhead url for each quest. It will tell you true or false if the quest is completed or not.