In the shop the “Free” tab for the app is grayed out and the Halo 5 forge doesn’t even show up. What gives?
Moved your topic to Halo App support for now OP - I believe if it’s greyed out then your PC might not be fully up to date.
You can try visiting here and see if your can download Forge, alternatively make sure that your copy of Windows is fully up to date with the Anniversary Update: https://support.microsoft.com/en-us/help/12387/windows-10-update-history
stckrboy is correct - it sounds like you are in a state where your PC is not fully up today. Make sure you have the Anniversary Update. If you do, then you might want to check your Windows update history - we have seen situations where people have an update that failed or otherwise didn’t get installed. This can also cause the download button to be greyed out in the store. Sorry for the inconvenience.