You’re used to seeing a horizontal (or sometimes vertical) list of your website pages at the top of your site.
However, on smaller screens (tablets and smartphones) your navigation may well have been replaced by what’s called a hamburger menu.
This is an icon showing 3 horizontal bars, a bit like this…
This isn’t an error, and is an intentional move to help make your site more mobile friendly by hiding your navigation items until they are needed. This gets over the issue of navigation labels being small and too close together, resulting in failed mobile friendliness checks.
When you click on the hamburger menu, it should reveal a list of your pages and posts. It may well be the same as the list of pages shown in your desktop menu, but not necessarily. If you or your developer thought that a different set of menu items might be appropriate for mobile users, then the list will be different.