When you add a new Link block within the Navigation block, the Link UI appears. However, if you click anywhere or generally move focus away, the Link UI disappears and its difficult to determine how to complete setting up the Link block. This leads to empty Link blocks with placeholder "Add link" text that doesn't do anything.

I think it could be helpful to improve the Link block's placeholder state. For example, if you unselect a newly added Link block we could show a button to reopen the Link UI and add a URL:

The experience for adding a Page or Post link could be adapted as well: