No type was found that matches the controller named ‘User’

In my case, the controller was defined as:

    public class DocumentAPI : ApiController
    {
    }

Changing it to the following worked!

    public class DocumentAPIController : ApiController
    {
    }

The class name has to end with Controller!

Edit: As @Corey Alix has suggested, please make sure that the controller has a public access modifier; non-public controllers are ignored by the route handler!