This error generally occurs when you try to display a Struts JSP, but the Struts ActionServlet has not been initialized properly.
Check your web.xml file and make sure
is specified in the xml that defines the action servlet.
If this is already there, then check your application server log for errors when the application starts up. Specifiacally, look for XML parsing errors. That's the most common reason for the Action servlet not initializing properly.