On a wide screen the menu is in view. On a phone it disappears, and there is no button to open it.
Visitors on a phone cannot reach the other pages. Google judges the mobile version of a site, so pages that only the desktop menu links to are harder to find.
Snaglist counts the links in the header and the nav at phone width and on a wide screen. When at least three are in view on the wide screen, fewer than two on the phone, and no menu button is found, it reports this. A page without any menu, like a simple landing page, is not reported.
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