Hey, I might of found a fix!
You've tried "Uninstalling Halo 5: Forge" right? Well, I found out it never really deletes the Halo 5: Forge folder when you "Uninstall" it, it seems to only get rid of the shortcut and some files that are required to start it.
When you download Halo 5: Forge again thinking it's uninstalled, it just updates the game with a few files on the Halo 5: Forge you had installed since who knows when.
So Here's what I did.
Start by deleting the Halo 5: Forge folder from:
C:\Program Files\WindowsApps\Microsoft.Halo5Forge (If you can't access Windows Apps, you have to change the owner to you)
[Right-ClickWindowsApps> Properties>|SecurityTab |> [ Advanced ] >Change Owner (Top-middle) >Type in your Microsoft Username > Hit Check Name>OK]
By Microsoft Username, I mean the Name that the login screen shows before you log on.
Once you have access, download this neat little tool called, Unlocker.
Select your bit version and download and install. (If you don't know what your's is) [Right-ClickStart>System] Should tell you there.
Once installed, goto:
[Right-Click on Unlocker.exe>Copy>Right Click on Desktop>Paste Shortcut>Right-Click & Rename to Unlocker]
[OpenUnlocker>ClickThis PC > Local Disk (C:) > Program Files > WindowsApps > Microsoft.Halo5Forge > OK]
Let Unlocker do it's thing, then after the folder is deleted, go ahead & re-download Halo 5: Forge, should work from there.
If it doesn't, either you need to Update your graphics card, your Windows 10 version or you didn't meet the requirements, but that's probably not the case.
(...unless that is the case then, uh... you'll need better specs or probably a better PC.)
Hope this fixes things for whoever finds this.