Halo CE PC "Required content not installed"

Missing Content
“The following required content is not installed: Halo: CE Campaign. Would you like to customize your installation now?”
This message appeared when I tried to play the campaign, but when I clicked ‘OK’ the Custom Installation page says I have both the campaign and multiplayer installed. I have uninstalled and reinstalled the 3.5GB of the campaign, but no luck. To preface incase it helps, when I first opened Halo MCC, steam ran a short update for it. I had no problems playing just yesterday before this update. I recall the in-game message listing fixes to Halo 2, but don’t remember anything about CE being changed. Any suggestions for a fix?