The three links at the top of each page need to be laid out across the top instead of down the page.  Like this:

The finished navigation bar

To do it:

Check that those styles are working on all pages.

*Note on browsers and sticky

A big problem with Web development is that there are no true standards.  Sticky is, at the time of writing this, part of CSS3.  However, it only now works in Firefox (it did work in Chrome for a while but then it did not again).  It is possible that sticky will later on be dropped totally and will only ever work in Firefox (if that).  In that case you might need another solution although the nav bar is still there in Chrome so it is a minor problem.  If sticky is not working for you in some browsers and you wanted the menu to stay visible all the time make these changes to use fixed instead:

Now the invisible div is fixed positioning (allowing it to stay visible) and the centering is done to the nav within that div.  This kind of fiddling and testing is normal in Web development.  That's why you should always test pages in a range of browsers.