Resolution 1: Please close the app, and open it again. If the problem persists try closing your internet browser and reopening it again. If the problem continues, try the solution from the following article:
https://support.microsoft.com/help/4035750
Resolution 2: If you changed your Dynamics 365 (online) URL after Microsoft Dynamics 365 App for Outlook was installed, you need to redeploy the app:
1. A user with the System Administrator role can redeploy the app to users by opening the Dynamics 365 web application and then navigating to the Dynamics 365 App for Outlook area within Settings.
2. Select the users who should have the app redeployed and use one of the Add App buttons to redeploy the app.
3. After the Status changes to "Added to Outlook", you can verify if the issue has been resolved. If you already had the app open, close and reopen it.