Since you are using only one category you need to edit the category's settings instead of the menu item.

If you already have this category mapped into another menu item you can add a dummy or blank category to force different settings.

What should i do with this category ?
Set my items belonging to two categories ?
I really don't get it.

If you only use this category in THIS menu item, simply change its category settings.

if this category is mapped by iteself to a different menu item too. One for example which shows all items, then you need to add another category in the settings (eg: one of its subcategories) to trick it so it respects the multi category settings.

Solved
Thank you !!

You 're welcome Yanni :)