go through following steps
1)check whether you have declared html taglib in your
jsp i.e,
<%@ taglib uri="ur tag lib location" prefix="html" %>
2)check whether ur giving correct form name in struts-config.xml
3)check whether the action in jsp
<html:form action=""> is referring to correct path in struts config.
If everything is correct and still u face problem, please post ur jsp and struts-config and actionform