There is no built-in function to completely flatten terrain in Roblox studio but there is a rather easy way to do it. I couldn’t find any other answers to this question other than people requesting it as a feature in Roblox studio.
Once you have the terrain you want to flatten, all you have to do is select the terrain, and go into the terrain editor tab → edit. Select replace, make sure the replace material option is set to box. and adjust the bounding box so that the height of the bounding box is just above the highest point in the terrain. Then put the bottom bounding below the ground, then move your width and length to however you want to be situated.
Next, depending on what your ground is ground is made of, you will replace the ground material with air. You may need to do this a few times if you have a biome with a lot of different materials to replace.
And that’s it. It’s really simple and it should have its own feature but until then this method works