You have the CSS class .position-sticky applied to your navigation (which I assume you added through the display panel dropdown), but there are no CSS rules defined for this class, nor can I find it in your CSS files.
To make the navigation sticky, you need to select the navigation, apply the class .sticky-top, and then your navigation will stick. Go to the properties panel and click the “+ Add class” button, then type in “sticky-top” in the search bar and click the “add class” button.
Fixed position forces an element to stay frozen to one spot relative to the viewport. Sticky allows an element to move within the viewport when you scroll until it reaches a certain point (usually the top) and then it “sticks” to that spot.