Problem with the category

Hi, I got a problem in my B.O :
I tried to create an under-category, so I add the main category as a parents for this one. The problem is the following --> It appear on the menu but when I follow the link I fall directly on a page which says “error 404”.

Here is the link of my website :

The page that I mentioned is "Actualités Automobiliste " under “Mobilité”.

Thanks for your help ;).