I recently faced this issue while I was trying to launch my ASP.NET MVC application from Visual Studio 2017 but I was unable to do that because Visual Studio was always showing me a popup with below error message.

Unable to start debugging on the web server. The remote server returned an error (404) not found.

enter image description here

Issue Root Cause

After some struggling, I found the root cause of the issue & it was some configuration settings in web.config file that was preventing Visual Studio from launching the application. And below was that setting - a requestFiltering tag.

        <requestLimits maxAllowedContentLength="2147483648" />
        <verbs allowUnlisted="false">
          <!--Allowed Verbs-->
          <add verb="GET" allowed="true" />
          <add verb="POST" allowed="true" />


Just add below tag inside verbs node. This will fix the issue & will allow Visual Studio to launch your application.

<add verb="DEBUG" allowed="true"/>