Bootstrap Issues

So i’m working currently on one of the Free Code Camp Projekts and i noticed that the Bootstrap responsive utilities don’t work for me so i set up a custom Codepen Projekt with the only purpose to create a heading, wich is invisible on all devices except small ones, but it doesn’t work. So can anyone help me?

It’s codepen. They are using some stupid alpha version of bootstrap now…
change it to:

Thank you very much :slight_smile: