Unable to create ocean bodies, lakes, and rivers.

I recently just started using UE (1 week now). I learned that UE has a built-in water plugin. The problem is that I cannot add any water bodies. The only water actor I can add is “water body custom”. At first, I thought this was because I was using a map that I imported from World Machine, so I recreated the map in the engine, but this didn’t fix the problem. So I opened a new level and inserted the ocean body, and it worked, but the river body didn’t. Am I missing something? I tried finding forums and tutorials, but they all suggest doing the same thing I’m doing. I made sure the “edit layer” box was checked when I was making the landscape. Is it possible to add water to a pre-existing landscape that’s been sculpted, or do I have to add water to a flat landscape before sculpting?