If the office 365 calendar not syncing, follow these steps. You must verify that your account's Calendar is activated to resolve this error. The first step is to tap the three lines in the upper left-hand corner of the Calendar in the Outlook app. Further, make sure the Calendar is selected under your email accounts. Your email address is not set up with Exchange ActiveSync if it is not visible in the list. You must remove your account and add it again, making sure not to utilize IMAP.