Why do I get the error message ‘Unable to start debugging on the web server…’ when I debug?

There is a possibility that there is no web.config file for the project or there is no Execute permission for the project.

If you already have a web.config file, double check it’s well-formedness.

If the web.config file is missing, add one to the directory that contains the ASP.NET application. You can copy one from the numerous samples applications available with the framework installation.

Make sure that the following section reads like this:

    <compilation 
         defaultLanguage='c#'
         debug='true'
    />

On the other hand, if your project already has a Web.config file, and the above section is set as specified, follow these steps to set the Execute Permissions property for the project folder:

1. Start Internet Services Manager, and then click the project that you are trying to debug.
2. Right-click the project, and then click Properties.
3. Click the Directory tab.
4. If None is selected in the Execute Permissions list, click Scripts only, and then click Apply