Slick Carousel Uncaught TypeError: $(...).slick is not a function

I found the solution myself later, so I placed it as an answer:

The error persisted every now and then when adding new function to my JS file. I ultimately stripped down my concatenated JS file and found out that there were two versions of jQuery being loaded, of which one was very, very old.

You failed to load the slick.js file. Add this script file https://kenwheeler.github.io/slick/slick/slick.js.

I updated your JSFiddle by adding an external file on left sidebar External Resources. Then it doesn't report the error: $(...).slick is not a function.

Recently had the same problem: TypeError: $(...).slick is not a function

Found an interesting solution. Hope, it might be useful to somebody.

In my particular situation there are: jQuery + WHMCS + slick. It works normal standalone, without WHMCS. But after the integration to WHMCS an error appears.

The solution was to use jQuery in noConflict mode.

Ex: Your code:

$(document).ready(function() { 
  $('a').click( function(event) {
    $(this).hide();
    event.preventDefault();
  });
});

Code in noConflict mode:

var $jq = jQuery.noConflict();
$jq(document).ready(function() { 
  $jq('a').click( function(event) {
    $jq(this).hide();
    event.preventDefault();
  });
});

The solution was found here: http://zenverse.net/jquery-how-to-fix-the-is-not-a-function-error-using-noconflict/