<div id="s" style="font-family: Tahoma;">
<span class="titlehead"><sup>Suggestion Box:</sup></span><br>
Got a suggestion to improve our working environment at Arqaam Capital, fill out the comment box below.<br>
If preferred, please leave the Name/ Department box empty to submit suggestions anonymously. <br>All entries will be checked on a regular basis and vetted by the HR team to ensure it is business appropriate.<br><br>

  <b>Comment:*</b><br>
  <textarea id="CommentCustom" required="" rows="4" cols="50"></textarea>
  <br>
  <b>Name / Department:</b><br>
  <input id="NameDept" type="text"><br><br>
  <input id="submitfeedback" style="color: white; font-size: 14px; background-color: rgb(152, 111, 11);" type="submit" value="Submit"><img id="customloader" style="display: none;" src="/resources/ajax-loader-circle.gif">

<br>
<div id="s" style="font-family: Tahoma;">
</div>
</div>

<div id="s" style="font-family: Tahoma;">
<span class="titlehead"><sup>Suggestion Box:</sup></span><br>
Got a suggestion to improve our working environment at , fill out the comment box below.<br>
If preferred, please leave the Name/ Department box empty to submit suggestions anonymously. <br>All entries will be checked on a regular basis and vetted by the HR team to ensure it is business appropriate.<br><br>

  <b>Comment:*</b><br>
  <textarea id="CommentCustom" required="" rows="4" cols="50"></textarea>
  <br>
  <b>Name / Department:</b><br>
  <input id="NameDept" type="text"><br><br>
  <input id="submitfeedback" style="color: white; font-size: 14px; background-color: rgb(152, 111, 11);" type="submit" value="Submit"><img id="customloader" style="display: none;" src="/resources/ajax-loader-circle.gif">

<br>
<div id="s" style="font-family: Tahoma;">
</div>
</div>

EDIT based on the reply i got from @BigRaj, i modified the script as follow:-

   $( "#submitfeedback" ).click(function(e) { 
   
    e.preventDefault();
   
   var namedept = document.getElementById("NameDept").value; 
    
   var commentcustom = document.getElementById("CommentCustom").value; 
    
   var itemType = GetItemTypeForListName("CommentBoxSource");
        var item = {
            "__metadata": { "type": itemType },
            "Title": namedept ,
            "CommentBoxComment": commentcustom
        };
        
        if(commentcustom.trim() === '' || commentcustom.trim() === ' ' || commentcustom.trim().length <= 5 ){ 
    $("#commentcustom").css("background-color", "rgb(256, 0, 0, 0.5)");
    $("#commentcustom").attr("placeholder", "Please enter value here");
        
  // reset textarea once clicked again
  $("#commentcustom").click(function() {
    $("#commentcustom").css("background-color", "white");
    $("#commentcustom").attr("placeholder", "");
    return;
  });

    return; //<--- This will prevent the ajax call if the Custommessage textarea is blank or only contains whitespace
  }
$("#customloader").show();
        $.ajax({
            url: "/_api/web/lists/getbytitle('" + "CommentBoxSource" + "')/items",
            type: "POST",
            contentType: "application/json;odata=verbose",
            data: JSON.stringify(item),
            headers: {
                "Accept": "application/json;odata=verbose",
                "X-RequestDigest": $('#__REQUESTDIGEST').val(getDigest())
            },
            success: function (data) {          

$("#customloader").hide();
                $('#s').find('input, textarea, button, select').attr('disabled','disabled');
                $( "#s" ).replaceWith( "<span style='color:rgb(114, 83, 8)'> Your feedback was submitted successfully</span>" );
            },
            error: function (data) {
                $("#customloader").hide();
                 alert(data);
            }
            
        });
        
});

 function GetItemTypeForListName(name) {
        return "SP.Data." + name.charAt(0).toUpperCase() + name.split(" ").join("").slice(1) + "ListItem";
    }
    function getDigest(){
  return $.ajax({
    type: 'POST',
    url: '/_api/contextinfo',
    dataType: 'json',
    headers: {
      Accept: 'application/json;odata=verbose'
    }
  }).then(function(results){
    return result.d.GetContextWebInformation.formDigestValue;
  });
}

but still i am getting the same error...