Set up two-factor authentication for your Apple ID.Make sure that your PC meets the iCloud system requirements.I suggest using the steps in support article: Get help using Outlook with iCloud for Windows, specifically: It's important to make sure that you can see your iCloud information in Outlook, so I'll be happy to help assist you with this. I see that you are having an issue with your new iCloud calendar appearing in Outlook. Thanks for using the Apple Support Communities. Outlook App For Mac Not Displaying Synced Calendar 2017.Outlook uses an.ost file only if the Exchange email account is configured to use Cached Exchange Mode. View the count under View statistics for this folder. On the General tab, select Show total number of items. Open the Calendar pane in Outlook, then right-click the Calendar folder. If the account is still not syncing, delete and re-add the account again on Mail Settings Manage Accounts Select the account, select Delete account from this device, and clicking the Delete option.Once the account is removed, select Add Account to add the account one more time. After completing the steps, the Mail app should start working again.