For one, I think you need to remove the “&request=GetCapabilities” part of your URL. Secondly, there’s an HTTP/HTTPS issue here: the WMS server doesn’t seem to support HTTPS properly which is a problem since your site is using HTTPS. Third and perhaps most importantly, when I just tried to look manually at the WMS output having fixed those issues, the WMS server seemed to encounter an error.
You can fix some of this but possibly not all of it.
The final point I noted, the error, isn’t really a problem, it’s just a symptom of the fact that you didn’t choose a layer when adding your WMS overlay. There’s only one layer in what you’re trying to use: sde:GISPORTAL.GISOWNER01.VACOUNTIES10
A sample request working with the layer selected: link