When navigating to the Salesforce plugin within the desktop version of Outlook, have you seen the following message and the plugin failed to load? "The app can take some time to load. You can wait, try closing the app and opening it again, or clear your settings and log in again."

This issue is likely due to the version of Outlook or Windows you are using. The Outlook Integration uses your default Microsoft web browser to launch the Salesforce add-in from desktop versions of Outlook. Salesforce is no longer supporting the Outlook Integration with Internet Explorer 11. This requirement is true for all add-ins built on that framework, not just the Salesforce add-in.

Firms NOT using Office version 16.0.11629 or later (i.e. Office 365 or 2019 one-time-purchase RETAIL only) and running Windows 10 version 1903 or later (e.g. Outlook 2016 and 2013) should make plans to upgrade to the latest version of Outlook and Windows to ensure support after December 31, 2022. These newer versions of Outlook use Edge instead of Internet Explorer 11.  You may also utilize Outlook on the web, although we do recommend upgrading if you are using Outlook 2016 or older.  

Please see these articles for more information:

Outlook Integration System Requirements 
Outlook Integration Internet Explorer 11 End of support

To check your version of Outlook see this article for more information:

What version of Outlook do I have?