"slider is not properly configured"

Hi guys,

I tried to set up the slider using the documentation for the theme. I started with 2 posts to try the functionality. I made a new categorie called “slider” and then in the slider options choose “slider” as categorie to be displayed. Then i only got a message on the homepage saying: “slider not properly configured”. Anyone has a good idea about this? And what are the best sizes for the slider images?

Thnx a lot and enjoy this day!