I wanted to send a ticket about this issue but either I can’t find the option to or I can only send tickets related to MCC or Infinite.
I’ve just gotten Halo Wars 2, never played any of the Halo Wars games before, I played it for 5 hours with a friend, we did a couple campaign missions on Normal difficulty as well as played a really long firefight that lasted about 60 waves and xbox says I’ve only played the game for 14 minutes and haven’t unlocked a single achievement.
Dangit! I’m sorry to hear that, man. Halo Wars 2 has been notoriously buggy, especially as of recently.
Try completing a short mission on Solo and see if you get the achievement. There’s not much evidence to support this being the reasoning, but Co-op might be interfering with your Campaign progress.
Just tried that to see if that was it, I did 2 missions Solo just to be 100% sure, and well… I still get no achievements.
There’s 1 difference though, I see on the xbox app that it has registered my actual playtime (6h 30min).
Not like that matters tho, I just want my achievements 
Yeah. I would too.
The nuclear option would be to uninstall and reinstall the game. Progress will be kept.
This has worked for me regarding another bug for Halo Wars 2 and other games with achievement issues.
Soo I just reinstalled the game, completed a mission again (solo) and still 0/126, all achievements Locked.
I’ve played some Spartan Strike through the Xbox app (which is also what I use to play HW2) and I was unlocking achievements for that game, which rules out the possibility that the app doesn’t grant any.
I unlocked an achievement 2 days ago, so I guess it works for me. I have no idea why it doesn’t for you, sadly.