The class or interface 'StyleSheet' could not be loaded!

1

I’ve tried everything and nothing works. I can’t figure this out. Before I even test the movie I get this error, “The class or interface ‘StyleSheet’ could not be loaded.” (I am not getting my xml content either, but I have not gotten that far.)

Here is the code that adamantly will not work.

stagegate_txt.wordWrap = true;
stagegate_txt.multiline = true;
stagegate_txt.html = true;
var sgStyle:StyleSheet = new TextField.StyleSheet();
sgStyle.load("Tab.css");
sgStyle.onLoad = function(success:Boolean) {
    if (success) {
        trace("Styles loaded:");
        var styles_array:Array = css.getStyleNames();
        trace(styles_array.join(newline));
    } else {
        trace("Error loading CSS");
    }
};
stagegate_txt.styleSheet = sgStyle;
var Del_str:String;
var Func_str:String;
var Role_str:String;
var Check_str:String;
Func_btn.onRelease = function(){
 stagegate_txt.text = Func_str;
}
Role_btn.onRelease = function(){
 stagegate_txt.text = Role_str;
}
Del_btn.onRelease = function(){
 stagegate_txt.text = Del_str;
}
Check_btn.onRelease = function(){
 stagegate_txt.text = Check_str;
}
var stagegate_txt:XML= new XML();
stagegate_txt.ignoreWhite = true;
stagegate_txt.onLoad = function(){
 var objXML = this.firstChild.childNodes;
 for (i in objXML) 
  stagegate_txt.text = objXML[0]; //This sends the Intro information to the TextAreaInstance.
  this.Func_str = objXML[0]; //I'll use this variable in a button to pass Intro content to the TextAreaInstance.
   this.Role_str = objXML[1];//This loads this button to pass Description content to the TextAreaInstance.
 this.Del_str = objXML[2]; //I'll use this variable in a button to pass Syntax content to the TextAreaInstance.
 this.Check_str = objXML[3];//I'll use this variable in a button to pass Exercise content to the TextAreaInstance.    
}
stagegate_txt.load("Stage0.xml");

Any help will be appreciated.

Thanks