On button click ajax is not working can anyone help me

Why are you using e.preventDefault(); in the start of click event. Remember

e.preventDefault();

always prevents the browser to stop executing the below lines of a specific event.

Change the code to:

$("#submit-btn").on("click",function(e) {
  
  $.ajax({
    url: "https://ivr.callxl.com/callXLWeb/SendingEmail",
    type: "POST",       
    contentType: "application/json; charset=utf-8",
    data: "comment="+$("#cmessage").val() ,
    dataType: "text",
 
    success: function (data, textStatus, jqXHR) {
      if (data.success) {
        alert("successfully sent");
      } else {
        // handle error here...
      }
    },
    error: function(jqXHR, textStatus, errorThrown){
      alert(jqXHR.responseText);
      alert("error in server");
      console.log("Something really bad happened " + textStatus);
      $("#errorResponse").html(jqXHR.responseText);
    }
  });

e.preventDefault();
});