This error occurs when the Struts ActionServlet has not been initialized prior to your calling a Struts JSP. There are generally two possible reasons for this:
1. You didn't put <load-on-startup>1</load-on-startup> in the ActionServlet's definition in your web.xml file.
2. The ActionServlet is throwing an exception when it tries to initialize. If this is the case, you will see a stack trace indicating what the problem is in the Application server's log when the application first starts up. The most common reason it might throw an exception is that your XML is not well-formed in the struts-config.xml file.