Like all other autofocus mechanisms ... some contrast is required in the region of the active AF sensor(s) in order for the mechanism to work.

If the Moon is in the sky, or there are some cloud with sharp outlines (like the billowy bits of a cumulus cloud, or a jet contrail), you should be able to focus on those. Do NOT under ANY circumstances attempt to focus on the Sun, you will very likely burn out both the camera and your eyesight!!!

Otherwise, focus on a distant object (a building roof, telephone pole or something similar, hundreds of yards away) then either hold focus by keeping the shutter release half pressed whilst recomposing or switch to manual focus mode.