Have you checked to ensure you have the latest versions of Teams and Outlook installed? Outdated versions might have compatibility issues or missing features. If they are updated, try reinstalling the Teams Meeting Add-in; Right-click the Windows Start button and select Settings > Apps > Installed Apps. Search for the Teams Meeting Add-in, click the three dots next to it, and select Uninstall and Reinstall it.
Have you checked to ensure you have the latest version of Teams installed. Outdated versions might not have the add-in or could have compatibility issues. Please try reinstalling the Teams app, this should force it to reinstall the Teams Meeting Add-in.
Have you checked for the Add-in Status in Outlook? Open Outlook and go to File > Options > Add-ins > In the Manage box, select COM Add-ins and click Go. Ensure the Teams Meeting Add-in is listed and checked.
Have you set Teams as the Default Chat App for Office? In Teams, go to Settings > General and check the option to “Register Teams as the chat app for Office” and Restart both Outlook and Teams.