首先更改脚本的顺序：

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.0/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/flatpickr/4.5.7/flatpickr.min.js"></script>

然后你在搜索flatpickr之前忘记了jQuery符号：

$( ".flatpickr" ).change(function() {
  alert( "Handler for .change() called." );
});

你必须在HTML代码下面初始化一个jQuery和flatpickr。你应该在最后用在身体上。像这样：

$('.flatpickr').flatpickr();
        $("#flatpickr-disable-range").flatpickr({
          disable: [{
              from: "2016-08-16",
              to: "2016-08-19"
            },
            "2016-08-24",
            new Date().fp_incr(30) // 30 days from now
          ]
        });
        $(".flatpickr").change(function () {
          alert("Handler for .change() called.");
        });

<link href="https://cdn.jsdelivr.net/npm/flatpickr/dist/flatpickr.min.css" rel="stylesheet"/>
      <div class="form-group">
        <input class="form-control flatpickr" id="set_exam_date" data-min-date="today" data-date-format="D F j, Y"
          placeholder="Select Date">
      </div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/flatpickr/4.5.7/flatpickr.min.js"></script>

你也可以使用javascript vanilla。例如，我在vanilla js上使用flatpick版本。

flatpickr(".flatpickr", {});

document.querySelector(".flatpickr").onchange = function() {        
        alert("Hello");
        };

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flatpickr/dist/flatpickr.min.css">
<script src="https://cdn.jsdelivr.net/npm/flatpickr"></script>
<input class="flatpickr" type="text" placeholder="Select Date.." readonly="readonly">

check with flatpicker document

flatpickr(".datepicker" ,{
    onChange:function (){
      //date change function hear
    }
  });