Wix Forms: OnWixFormSubmitHookError => changes to fields are not valid

@yisrael-wix
This my code with console.log

   $w("#wixForms1").onWixFormSubmit((event) => {
    console.log("Inside on submit");
    console.log ("On Entry - ", event.fields);
    let index = $w("#dropdownCourseName").selectedIndex;
    let dropdownoptions = $w("#dropdownCourseName").options;
    console.log ("Dropdown Label :  ", dropdownoptions[index].label," Value: ", dropdownoptions[index].value);
    let coursename = dropdownoptions[index].label;    // Set to readable label 
    // let coursename = "FIRST NAME"                    // To Set another text field 
    // let coursename = dropdownoptions[index].value;     // Set to original unreadable options value
    //      let coursename = dropdownoptions[index+1].value; // set to another unreadble options value

    console.log ("Coursename ", coursename);
   // console.log( "Length", event.fields.length);
    for (var i=0; i < event.fields.length; i++) {
   //       console.log ("Event field", event.fields[i]);
         if (event.fields[i].id === "dropdownCourseName"){
           console.log ("Event field", event.fields[i]);
           event.fields[i].fieldValue = coursename;
    //        event.fields[i].fieldValue = "";          // change fieldValue to ""
    //        event.fields[i].fieldValue = " ";         // Change fieldValue to " "
         }
         /** 
      if (event.fields[i].id === "input25"){
             event.fields[i].fieldValue = coursename;
           
         }
           **/ 
    }
       console.log ("On Exit - ", event.fields);
    return event.fields;
  })  

This is the console.log messages


This is expanded On Entry console.log

This is the expanded On Exit console.log


The form submission message: