Many (if not all) Full Site Editable (FSE) themes do not provide the familiar WP access to create or edit Menus – that found by going to Appearance>> Menus or Customize >> Menus. That functionality is found by going to Edit Site and making changes to the template or template part where the menu is displayed. Typically, this will be in the Header template where you will find a Navigation Block.

In the screenshot below, you can see that the Header template is displayed (for more information on templates and template parts see Full Site Editing) and that List View has been selected (for more information about using List View, see Block “List View”) and the Navigation Block has been expanded. You can edit the Navigation block by either using List View (found on left) or on the Navigation Block (on the right) where there is a convenient plus (+) sign to add a Page Link block to your menu. Or if you want to edit a menu item, click on the item icon and make your changes.

If this feels too complicated, there is another way that handles FSE Menus by bringing back the Appearance >> Menus functionality by activating a plugin called Classic Menu in Navigation Block.  Create a menu and save it. You will still need to edit the Header Template, but only once. If you choose this option, click on the three dots on the Navigation Block in your Header and can choose the name of your menu you created and after saving, you will not need to go to Edit Site to add or edit menu items. Just go to Appearance >> Menus make your changes and they will automatically be displayed in your header.