If someone has run a lockdown or other security tool on the Web server, the
DEBUG method may have been removed.
Go into IIS manager to the Web application’s properties and Directory >Configuration (button) > Mappings > Application mappings.
Check that for .aspx extensions, you allow GET,HEAD,POST,DEBUG
Of course you need to be a member of the Debuggers group on the remote server.
To resolve error
- Right-click the Project Name in Solution Explorer, and then click Properties.
- In the left pane of Properties, click to select Configuration Properties.
- Click to select Debugging.
- In the right pane, change the value of Enable Unmanged Debugging to True (on the drop-down list).
For more details refer PRB: Error Message: Unable to Start Debugging. Unable to Start Program …