Can't make a Skybox

Hi, I've been trying to make my own skybox and have run into some trouble.

I was following this, so i'd made the textures, and the material. But when trying to edit the material to add the textures it just gave me this

Any help would be appreciated, thanks.

EDIT: It appears I have managed to fix it, I feel really stupid for not trying but i think it's because I opened a new scene.