sharepoint 2010 error the web application at could not be found

1. The code is executed on a different machine - The SharePoint object model (except the Client API) requires to be run on the SharePoint server itself. It is not possible to run the application on a server which is not within the same SharePoint farm the code is trying to access.
2. Insufficient Rights on the site collection - The code is executed in context of an account which does not have read permission on the site collection
3. Incorrect Url being used - Verify that the site works correct in a browser and double check that the server is correct registered in the AAM settings
4. Incorrect bitness - The SharePoint object model needs to be executed with the same bitness as the operating system. That means you cannot use the SharePoint object model in a 32-bit application if the Operating System and SharePoint are installed as 64-bit version. Ensure to compile the project using the correct bitness (64-bit on a 64-bit machine vs. 32-bit on a 32-bit machine)
5. Incorrect .NET framework version -  Ensure that the project is configured to use .NET 3.5. .NET 4.0 cannot be used with the current versions of SharePoint
6. Run your application with Administrator account.(Open Visual Studio as Administrator)