I have a very weird problem. If I add a new menu item, I can do it without any error in the menu configuration page, but the item isn't showed in the menu in this page neither in the menu. If I check menu tables in mysql, the items are added. If I try to add another item, I can create a child of the other in the menu structure, but the item isn't showed.
There are a couple possibilities for why your menu link isnt showing.
Have you checked that the menu being rendered is the same as the one you added the link? Typically, menus are rendered via Drupal's block layout settings. (Admin > structure > block layout) You can check that the menu rendered on your site is the same as the one you are adding the links to.
If using the Menu Block module, check that your menu block settings aren't set to display a fixed level of the menu or fixed trail.
Is the link you've added to a page or view on the site that has permission restrictions? This sometimes happens when trying to add custom admin routes.
If the settings above are correct, try flushing the drupal caches when updating menu links before checking on the front end.
In the case none of the above works, and you are on a development instance of the site, try switching to a standard theme (like Bartik) to quickly determine if somethingin your theme is preventing the links from rendering.