Is there a setting that I have to switch on first? I could not achieve split screen on a macbook air running El Capitan

I tried to get a split screen in every way that I found described, but I do not succeed. Where the + sign is supposed to show up, I always see the forbidden sign.

My gear is a macbook air with no additional stuff, running El Capitan.

Is there something I have to do first to enable split screen?