local terrainRegion = workspace.Terrain:CopyRegion(Region3int16.new(Vector3int16.new(-1755.56, 143.739, 457.646),Vector3int16.new(-795.289, -14.17, -451.93)))
workspace.Terrain:Clear()
wait(5)
workspace.Terrain:PasteRegion(terrainRegion, Vector3int16.new(-1755.56, 143.739, 457.646), false)
So not quite sure what is going on, no error in the output, it successfully clears all terrain but then it won't paste it back.
I tried using MaxExtents and it worked, so I have no idea why my vectors won't work.

5 comments
Are you sure that the region specified is correct and contains terrain?
I'm pretty sure they are, what would be the best way to double check?
Make two bricks at the coordinates that you define here in your script. Think of them as the top left and bottom right corners of a box. The space between them is what is referenced. That should help you visualize what is contained in your Region3int16.
Yes, I have done that, terrain is inside of it.
So it seems to be all messed up, the "box" is different from the terrain that pops up???