Menu creation problem
A Drupal menu item can point to external URLs, URLs to files in the filesystem or an internal path. The latter has to exist before creating the menu item and access permissions are needed for the user creating it.

A path can point to a node or any other entity, or any path provided by a module (e.g a view from the Views module). Virtually any path can have any number of aliases but it's worth using the path in the menu items as aliases may change throughout the lifetime of the site, paths are less likely to change.