I contacted Microsoft Support about this issue and they said that I needed to ask for developer help on here.
I purchased Halo Wars 2 Ultimate Edition from amazon and successfully activated it onto my Microsoft account on August 18th 2018. I am able to download and play Halo Wars Definitive Edition and Halo Wars 2 on my xbox one. I am unable to download Halo Wars 2 from the Microsoft Store but I am able to download and play Halo Wars Definitive Edition from the Microsoft Store.
My computer does meet the minimum and recommended specifications. I have:
i7-7820X
32GB of RAM
GTX 1080 with 6GB of RAM
1.2 TB of free storage space
Windows 10 pro 64-bit version 1803
This problem also occurs on my Surface Book with an i7 and a GTX 1060.
I am able to see Halo Wars 2 in “My Library” but when I try to download it, I only have the option to “Install on my devices”. When I click this option, it only allows me to download it on my Xbox One’s. On the Halo Wars 2 page, it also says “See “Available On” below for devices this product runs on. It won’t run on this one.”.
I have tried, and been unable to, download Halo Wars 2 from: the Microsoft Store, the Xbox app, and the Halo app. I have ensured that I am logged into the same Microsoft account on all of my devices and apps.
Here are some screenshots I have of the issue: Imgur: The magic of the Internet