Use the “index.html” minimal template file from this sketch below instead: 
index.html:
<script defer src=https://Unpkg.com/p5></script>
<script defer src=sketch.js></script>
Other alternative CDNs: 
As a last recourse, we can grab “p5.min.js” directly from their site: 
p5js.org/assets/js/p5.min.js