I am trying to create a new menu link. But the system doesn't allow me to do so.
The path 'New' is either invalid or you do not have access to it. The path 'testss' is either invalid or you do not have access to it.
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.
Try just using the node and its id like "node/id#", it will pick up the alias if it has one.
You first need to create some content which has the path "new". Create a node and in the "URL Path Settings", give it an alias "new". Now, you should be able to create menu item with that link.
Try creating a content eg article via node/add/article and use the url for the content aka node which is like node/[node:id] ; node:id is the numeric id number of the created content.
Drupal needs a valid path to create menu items. The link or path that you provide should exist or should be accessible.
You can go through these articles for a clear idea: https://www.drupal.org/documentation/modules/menu https://www.drupal.org/node/788972
Good luck