Water is supposed to flow into the pit from the ground so it can be pumped up and out. That's why it's open underneath.
There are 2 problems with this, though. The dirt doesn't provide a stable, level surface for the pump. Also, it increases the chances of dirt getting into the impeller of the pump.
You can get a pe basin, but make sure water has a way to get in. I'd also sit the pump on 2 bricks or something else that is dirt-free, heavy, and stable/level.