I am not sure why you are getting these results. The Visual Designer has worked for a solid release. I just did you what you asked in VS2019 (albeit with C#).

I am not sure why you are getting these results. The Visual Designer has worked for a solid release. I just did you what you asked in VS2019 (albeit with C#). File/New Project. Select Winforms App for .NET Core. A new form comes up. Click to Toolbox on the left. Drag a button to the form. Double click on the button to handle the click event. It's that simple. No errors.