I have been trying to delete the Halo MCC game files folder from my laptop, and I have the necessary permissions to do so however whenever I try to delete the folder it says it is unable to due to the folder being open in System. Any help would be appreciated.
Its most likely saying that the file is open by the user SYSTEM, which indicates that a service has a lock in it.
It is likely that this game has a DRM protection or anti-cheat service running. The right way to remove the folder is to first uninstall the game using the installer, so all dependencies are uninstalled, then you can delete any leftover folders.
Deleting in safe mode should work, while ur in safe mode, try finding services that interfere or actively use/lock the game files and disable them. You might also wanna try takeownershippro.
TLDR: close applications and services that may be using whatever you want to delete(make sure to system restore point first)