I am trying to figure out what could be the issue. I need the exact quest name that you are having issues with or the name of the guide you are using.
Are you on an North America or Europe realm?
Looks to me that you need to exit World of Warcraft and click Settings, then click Repair, click Okay x3 times, then click Okay. Click reinstall if it doesn’t install automatically.
Enter the game with only Dugiguides enabled.