Could your shortcode function be running or loading before Slick.js has loaded?
Simple solution might be to load slick in the header by changing the last parameter from true to false, but this isn’t really the ‘correct’ way as it will impact performance slightly.
If your shortcode function is in a seperate file loaded by enqueueing then add slick as a dependancy.
If it’s in the HTML so loaded from a PHP file, move it to a seperate JS file, enqueue it with admin_enqueue_scripts and again add Slick as a dependency.