$.ajax is not a function

$.ajax is not a function

Hi I just started using 
 jQuery JavaScript Library v3.1.1

and i get 2 errors
jQuery.Deferred exception: $.ajax is not a function (15:41:01:532 | warning)
  at Files/Web_UI/js/jQuery-3.1.1.js:3688
Uncaught TypeError: $.ajax is not a function (15:41:01:535 | error, javascript)
  at Files/Web_UI/js/jQuery-3.1.1.js:3697


I get it when I run

    $.post("ajax.php",
            {action: "InitLoad"},
            function (data) {
                alert(data); 
            }, "json");

in 

script type="text/javascript" src="js/js_Functions.js"> /head> body > div>TODO write content button onclick="init()">Click here script> $(document).ready(function () { init(); }); /script> /body> /html> " -->


I need help please !!!!!!!!!
Thanks in advance