ok so if i try to run this script that will instance a new part at a random vector3 position, it gives me the error in the title.
luis = Instance.new("Part",workspace)
while true do
luis.Size=Vector3.new(5,1,5)
workspace.Part.Position=Vector3.new(math.random(138.5,252.5), (-18.5), math.random(-137.5,-252.5))
workspace.Part:Destroy()
wait(2)
end

3 comments
The error occurs when the (minium) 1st value of the parameter - being math.random- is bigger than the second. So, all you'd have to do, is make the 2nd value bigger than the first. That should fix your problem.
workspace.Part.Position=Vector3.new(math.random(138.5,252.5), math.random(-19,-18.5), math.random(-137.5,-252.5)) is my new and fixed string but it still gave me the error
Try putting (-137.5,-252.5)), (-19,18.5) and (138.5,252.5)