Открываем консоль, а там... а там... а там вот что:
Mixed Content: The page at 'https://jsfiddle.net/' was loaded over HTTPS, but requested an insecure script 'http://cdn.jsdelivr.net/npm/slick-carousel@1.8.1/slick/slick.min.js'. This request has been blocked; the content must be served over HTTPS.