How can I resolve the flex slider?

Can anyone help me figure out why am I getting this error?

Uncaught TypeError: jQuery(…).flexslider is not a function

    jQuery(document).ready(function() {
    jQuery('.slider').flexslider();

at HTMLDocument.<anonymous> (custom-mega-menu.js:4:11492)
at i (custom-mega-menu.js:2:33554)
at Object.fireWith [as resolveWith] (custom-mega-menu.js:2:34521)
at Function.ready (custom-mega-menu.js:2:36662)
at HTMLDocument.K (custom-mega-menu.js:2:37059)

the code I have:

I tried googling and youtube but I can’t find something specific to help figure it out.