Error: Unable to start debugging on the web server. See help for common configurations errors.Running the web page outside of the debugger may provide further information.
Description: This error occurs while running or debugging an application using Visual Studio 2010. The reason behind this problem is because of .NET framework version configured for your project and the IIS Default App Pool or the Pool that configured for your application is not matching correctly. For example: .NET framework configured for project is v3.5 but at application pool level it might be v4 or higher. At this time VS was unable to find out which pool should pick for debugging.
Solution: Go to Application Pool using, click Basic Settings under Edit Application Pool and remap the correct .NET Framework version.
Example: If project using v3.5 then remap .NET Framework version to v2.0, rerun the project again.