Users that are on a combination of Windows and Office versions that still use Internet Explorer to render add-ins may run into this error. See this article from Microsoft to learn more about how Microsoft determines which browser they choose when you use add-ins.
Steps to Resolve
The preferred method is to update your Microsoft 365 environment to a version that does not require Internet Explorer.
Per the Microsoft article referenced above users should upgrade to Windows 10 version 1903 or higher and Microsoft 365 version 16.011629 and higher.
Enable Protected Mode in the Internet Properties
If upgrading is not an option then enable protected mode in Internet Explorer. Even thought your default browser is not Internet Explorer note that Microsoft leverages Internet Explorer for users on specific combinations of Windows and Office versions.
- Close Outlook and all your browsers.
- Open Internet Explorer.
- Click Tools.
- Click Internet Options.
Alternatively, for steps 2 to 4; you may also access the Internet Properties through the steps below:
- Press 'Windows logo' key + R on your keyboard.
- Enter inetcpl.cpl ten press Enter key.
- Click Security tab.
- Select Internet Sites zone.
- Select Enable Protected Mode (make sure box is checked).
- Click OK.
- Restart Internet Explorer and Outlook.
- Try to reproduce the issue
- If the issue persists reboot your computer and try again