That request doesn’t look like it’s reaching ServiceStack for whatever reason, I’m assuming you have a conflict somewhere, do you have a folder called /api? or have you configured ServiceStack to run on /api? If you have you shouldn’t prefix your route with /api.
There looks like there’s a conflict preventing the request from reaching ServiceStack, I can’t see your configuration so I’m guessing for different things to check. If you have WebDav enabled, you can try removing the WebDav module.
For DELETE requests you can populate params via QueryString, e.g: