I have re-written the instructions below from other posts. Follow them, I know they work for Outlook 2013 and 2016 on Windows 10. I have fixed up a half a dozen computers.
Open the iCloud panel and sign out
Open outlook and then follow the instructions below:
1. File-> Account Settings-> Account Settings…-> Data Files tab
2. Select the iCloud data file and press Remove.
3. If you also connected to the iCloud account via IMAP, remove this account.
4. Close Outlook.
5. Press Windows key + R.
Copy and paste the following command in the open box,
Outlook.exe /resetnavpane
6. and then press Enter. Note there is a space between Outlook.exe and /resetnavpane.
7. Open the iCloud Control Panel via the Start Menu and uncheck all the options: Mail, Contacts, Calendars, and Task with Outlook.
8. Download and install the latest version of iCloud from the Apple website.
9. Reboot when being prompted to do so by the iCloud installer.
10. Open the iCloud Control Panel via the Start Menu sign in with your apple ID and select the option: Mail, Contacts, Calendars, and Task with Outlook.
11. Start Outlook and let iCloud resync your data.