Give your problem, I recommend you can follow the steps below to troubleshoot your issue:
1. Open registry editor and go to “HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Addins”
to check whether your add-in is installed correctly. By default, the value of LoadBehavior is “3”, with which the VSTO Add-in will be loaded when you launch Outlook, if LoadBehavior value of your add-in is not “3”, you can try to change it to “3”.
2. Start Outlook to see whether the add-in works well. If the add-in shows up at “Active Application Add-ins” and the status of “Load Behavior” is
Load at Startup, it should work well.
3. If the problem still exists, you can check the error via right-clicking
This PC to Properties, and then go to Advanced system settings >
Environment Variables > New to add a new system variable, the variable name is “VSTO_SUPPRESSDISPLAYALERTS”, the variable value is “0”. In addition, open the registry editor and go to “HKEY_CURRENT_USER\Software\Microsoft\Office\Outlook\Addins”
to set the LoadBehavior of your add-in to be “3”. Then when you restart Outlook client, there will be pop-up dialog describing what the error is, and share the screenshot with me.