not all skies light your map afaik… you should add lights, an ambient key in worldspawn, or compile without light settings (which gives you full brightness everywhere though), depending on what you want…
if you’re just testing no light compile will probably do (except if your testing lighting ofcourse)
Adding an ambient key in worldspawn (key: ambient, value: around 10, 12, experiment with it) will also light up your map without using light entities…
or you could use light entities. But without an ambient key, anywhere where there’s no light entity it will be pitch black, and near a light entity full bright ^^