If Outlook still gives you the outlook set folders cannot be opened
Use Taskbar shortcut to start Outlook
Many users tend to pin the most used applications to their Taskbar in order to launch them quickly. According to users, you can avoid The set of folders cannot be opened error message simply by starting Outlook using its Taskbar shortcut.
To do that, follow these steps:
Make sure that Outlook is pinned to your Taskbar. If not, locate Outlook shortcut and drag it to your Taskbar.
Locate Outlook shortcut on your Taskbar and right-click it. Choose the New E-mail message or Outlook 2016 option from the menu.
After doing that, Outlook should start without any problems or error messages. Keep in mind that this is just a workaround, so you’ll have to repeat this solution every time you want to start Outlook.