It is a bit unclear to me what you cannot "see" in sidebar layout, since you are not using sidebar layout. Do you mean you cannot
CLICK it in the sidebar layout? The sidebar layout has a different logic than the topbar layout, because CHILD-menu items will display in the menu on CLICK of the parent menu ... Therefore, in the sidebar layout, if a page has CHILD pages, clicking the PARENT page is reserved to TOGGLE the display of the child pages in the menu. After all, the visitor will still get to where they want in the child pages.
If you want the visitor to be able to CLICK the "Kroatien Istrien" item in the sidebar, your option is to go to page settings > menu > Child pages >
Hide child pages from menu. This will force the visitor to first enter this category parent page "kroatien Istrien", where they can then click to visitor child pages.
This is a logical difference between the sidebar and topbar menu. Since the TOPBAR menu displays child-pages on mouse HOVER, the parent item itself is clickable. In the SIDEBAR menu however, which does not display child-pages on mouse-hover, CLICK is reserved for displaying child-items.
Your opens are to either "Hide child pages" for the sidebar menu, or use the topbar menu.