It is not possible for apps script to read the value of a cell that is still currently being edited. You might want to consider the following workarounds.
(Option 1)
Replace your Add Data button with a checkbox and use onEdit(e) simple trigger to call your function. When a checkbox is being checked/unchecked, it makes its cell active. Hence your 5th data entry can be read successfully.
Sample Code:
function onEdit(e){
  var ss = e.source;
  var cell = e.range
  if(ss.getActiveSheet().getName() == "Form" && cell.getA1Notation() == "B9" && cell.isChecked() == true){
    //Add data to form
    Adddataform();
  }
}
function Adddataform() {
  var ActiveSheet = SpreadsheetApp.getActiveSpreadsheet();
  var FormSheet  = ActiveSheet.getSheetByName("Form"); //Form Sheet
  var DataSheet = ActiveSheet.getSheetByName("Data"); //Data Sheet
  //Input Values
  var values = [[FormSheet.getRange("B4").getValue(),
                 FormSheet.getRange("B5").getValue(),
                 FormSheet.getRange("B6").getValue(),
                 FormSheet.getRange("B7").getValue(),
                 FormSheet.getRange("B8").getValue()]]; 
 
  DataSheet.getRange(DataSheet.getLastRow()+1, 1, 1, 5).setValues(values);
  SpreadsheetApp.getUi().alert("Data add successfull")
  ResetForm ()
}
// function resetByRangeList_(FormSheet,Formcells){
// FormSheet.getRangeList(Formcells).clearContent() 
// }
//............................
//Clear Form
function ResetForm() {
  var ActiveSheet = SpreadsheetApp.getActiveSpreadsheet();
  var FormSheet  = ActiveSheet.getSheetByName("Form"); //Form Sheet
  var Formcells = ["B4","B5","B6","B7","B8","B9"];
  resetByRangeList_(FormSheet,Formcells);
}
function resetByRangeList_(FormSheet,Formcells){
  FormSheet.getRangeList(Formcells).clearContent(); 
} 
What it does?
- Whenever a cell is being modified in your spreadsheet, onEdit() will be called. Check if the modified cell is Form!B9and if the checkbox was checked.
- If the checkbox was checked, Add the data to the Data sheet by calling Adddataform()
Note:
- Removed the undefined saveData()insideAdddataform()
- Include Cell B9 in the resetByRangeListto reset the checkbox
Output:

(Option 2)
You can use Custom sidebars or Custom dialogs as your form.
Sample Code:
(Code.gs)
//CREATE CUSTOM MENU
function onOpen() { 
  var ui = SpreadsheetApp.getUi();
  ui.createMenu("My Menu")
    .addItem("Sidebar Form","showFormInSidebar")
    .addItem("Modal Dialog Form","showFormInModalDialog")
    .addToUi();
}
 
//OPEN THE FORM IN SIDEBAR 
function showFormInSidebar() {      
  var form = HtmlService.createTemplateFromFile('form').evaluate().setTitle('Form');
  SpreadsheetApp.getUi().showSidebar(form);
}
 
 
//OPEN THE FORM IN MODAL DIALOG
function showFormInModalDialog() {
  var form = HtmlService.createTemplateFromFile('form').evaluate();
  SpreadsheetApp.getUi().showModalDialog(form, "Form");
}
 
//PROCESS FORM
function processForm(formObject){ 
  Logger.log(formObject);
  var ActiveSheet = SpreadsheetApp.getActiveSpreadsheet();
  var DataSheet = ActiveSheet.getSheetByName("Data");
  Logger.log(formObject.invoiceNumber);
  Logger.log(typeof formObject.invoiceNumber);
  var values = [[formObject.fleetNumber,
                formObject.date,
                formObject.invoiceNumber,
                formObject.jobNumber,
                formObject.operation
                ]];
  
  DataSheet.getRange(DataSheet.getLastRow()+1, 1, 1, 5).setValues(values);
  SpreadsheetApp.getUi().alert("Data add successfull");
}
(form.html)
<!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <!-- Bootstrap CSS -->
    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.5.3/dist/css/bootstrap.min.css" integrity="sha384-TX8t27EcRE3e/ihU7zmQxVncDAy5uIKz4rEkgIXeMed4M0jlfIDPvg6uqKI2xXr2" crossorigin="anonymous">
    <title>Sample Form</title>
  </head>
  <body>
    <!-- Option 1: jQuery and Bootstrap Bundle (includes Popper) -->
    <script src="https://code.jquery.com/jquery-3.5.1.slim.min.js" integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj" crossorigin="anonymous"></script>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@4.5.3/dist/js/bootstrap.bundle.min.js" integrity="sha384-ho+j7jyWK8fNQe+A12Hb8AhRq26LrZ/JpcUGGOn+Y7RsweNrtN/tE3MoK7ZeZDyx" crossorigin="anonymous"></script>
    <form id="myForm" onsubmit="handleFormSubmit(this)">
      <div class="form-group">
        <label for="fleetNumber">Fleet Number</label>
        <select class="form-control form-control-sm" id="fleetNumber" name="fleetNumber" required>
          <option value="" selected>Choose...</option>
          <option value="Number1">Number 1</option>
          <option value="Number1">Number 2</option>
        </select>
      </div>
      <div class="form-group">
        <label for="date">Date</label>
        <input class="form-control form-control-sm" type="text" class="form-control" id="date" name="date" placeholder="mm/dd/yyyy">
      </div>
      <div class="form-group">
        <label for="invoiceNumber">Invoice Number</label>
        <input class="form-control form-control-sm" type="text" class="form-control" id="invoiceNumber" name="invoiceNumber">
      </div>
      <div class="form-group">
        <label for="jobNumber">Job Number</label>
        <input class="form-control form-control-sm" type="text" class="form-control" id="jobNumber" name="jobNumber">
      </div>
      <div class="form-group">
        <label for="operation">Operation</label>
        <input class="form-control form-control-sm" type="text" class="form-control" id="operation" name="operation">
      </div>
      <button type="submit" class="btn btn-primary">Submit</button>
    </form>
    <script>
    // Prevent forms from submitting.
    function preventFormSubmit() {
      var forms = document.querySelectorAll('form');
      for (var i = 0; i < forms.length; i++) {
        forms[i].addEventListener('submit', function(event) {
        event.preventDefault();
        });
      }
    }
    window.addEventListener('load', preventFormSubmit);    
        
        
    function handleFormSubmit(formObject) {
      google.script.run.processForm(formObject);
      document.getElementById("myForm").reset();
    }
  </script>
  </body>
</html>
What it does?
- Create a custom menu which will be displayed whenever you open your Google Sheets

- Create an html form, use google.script.run to allow HTML-service pages to call server-side Apps Script functions. In this example we will call - processForm(formObject)using this- google.script.run.processForm(formObject);syntax once the form was submitted.
 
- Add new row data based on the parameters provided during form submission. 
Output:


Additional Reference: