You need to have installed Microsoft Visual Studio Tools for Office (VSTO). VSTO can be selected in the Visual Studio installer under Workloads > Web & Cloud > Office/SharePoint Development. After that create a generic .NET project and add a reference to Microsoft.Office.Interop.Excel via ‘Add Reference… > Assemblies’ dialog. Application excel = new Application(); Workbook … Read more
Here’s a bit more detail on RobertG5’s solution (too long for a comment): The problem was that the add-in had been hard disabled by Outlook. As I’ve learned, that’s something different than the “usual” not-loading scenario. The key to realizing this was to notice that the add-in did not show up under Inactive Application Add-Ins, … Read more
You need to re-add that certificate to your machine or chose another certificate. To choose another certificate or to recreate one, head over to the Project’s properties page, click on Signing tab and either Click on Select from store Click on Select from file Click on Create test certificate Once either of these is done, … Read more