Take out the bootstrap link in the Settings > CSS section and you should be set. For some reason Codepen’s defaulted to the alpha version 4 build of Bootstrap (CSS & JS) and it may cause issues. I didn’t read the docs but navs changed quite a bit; if you stay with bootstrap 4, probably the html classes for the navbar or items are different now from the way you have them. https://forum.freecodecamp.com/t/codepen-changed-the-default-bootstrap-version/85235
It’s ok to be at the bleeding edge of changes, but it does make learning more difficult when you are trying to get help from the internet. I would go with version 3 (added manually to codepen, the way you did in your link to stylesheet) unless you’re specifically trying to learn bootstrap 4.