The community member is having an issue duplicating a slot in Webstudio, as the system is reporting a missing NavigationMenuViewport. Other community members suggest that the viewport container in the navigation menu has been deleted, which is causing the issue. They provide a project link and advise the original poster to try deleting the navigation menu, as that should fix the problem. The original poster eventually resolves the issue, but it's not explicitly stated how they did it.
I opened your project. I see Sheet with navigation menu inside, it is not default, somebody put it there Navigation menu should have Viewport Container, I don't see it there You somehow managed to delete it, so I am asking how. If you delete this navigation menu inside of Sheet, all will be fixed