Unable to build custom action named '<name>'; InstallerClass property is only valid for assemblies
The specified custom action cannot be built because the InstallerClass property is set to true and the custom action is not an assembly. The InstallerClass property must be set to false for any custom actions that are not assemblies.
To correct this error
Double-click the error message in the Task List to go to the custom action in the Custom Actions Editor.
In the Properties window, select the InstallerClass property and set it to false.