Halo MCC on game pass not allowing me to play the games.

Hi. My name is Brilliant, an independent advisor, happy to assist.

Here are a few troubleshooting tips you can try to resolve your query, including some that you may have already tried:
- Restart your device.
- Update Windows.
- Update your device drivers.
- Update the Xbox and Gaming Services apps.
- Sign into the Xbox app and Store using the same account.
- Remove and reinstall Gaming Services.
- Make sure your date and time is correct.
- Disable your antivirus.
- Use the app troubleshooter.
- Repair and reset the Xbox app.

For more details on the above and more troubleshooting tips, kindly visit: https://support.xbox.com/help/games-apps/troubleshooting/troubleshoot-games-windows-10

I hope this helps resolve your query.

I bought Halo MCC on disc a while ago and it just updated, I was able to play reach on it once, but now every time I try to open the game it says that I need to play on an account that has a subscription and/or an account that has the updated game
pass, but I bought the game on disc? Why isn’t it allowing my to play a game I bought?

Reach isn't included with Halo Master Chief Collection, it's paid DLC for Halo MCC, but it's also part of Game Pass. Did you have an active Game Pass subscription or active trial when you tried out Reach last time?

Or are you getting the same message whenever you try to play any other part of Halo MCC too?

As Halo MCC was added to game pass Im playing all the old Halo games again.

Also playing Manual Samuel

Also this week I passed 300k Gamerscore and 15000 individual achievement unlocks

Same, games recently downloaded on Ultimate Game Pass aren't working for me (Halo 5, Halo MCC, Hue, PES2019) with that same error code, but older downloaded games are working.

For me my favourite game on game pass would have to be Halo 5, I think it's great that MCC got added too.