This error occurs when you try to display a JSP before the Struts ActionServlet has been initialized and is active. The causes for this error are usually either:
You failed to specify <load-on-startup>2</load-on-startup> for the Struts ActionServlet in your web.xml file or
You did specify the above, but the Struts ActionServlet didn't initialize properly because of an error. Check the log file entries for the time period when the Server first starts up to see if it initialized properly or
You accessed a JSP page directly without going through an action
When the Struts ActionServlet throws an exception while initializing, the most likely cause is that it was unable to parse the XML in your struts-config.xml file either because it's malformed, or it doesn't follow the DTD. If you look at the log file entries that occur when the server is first starting, sometimes you will find an error message telling you specifically what's wrong with the config file and also telling you the line number in the file where the error occurs.
Also, check to see that you have all the necessary jar files in your WEB-INF/lib directory. If you don't know for sure which ones you need, just throw in all the jar files contained in the lib folder in the Struts downloaded materials.