now why can't I do this? "bad argument #2 (interval is empty)"

math.random() expects integers to be given in ascending order, so the smaller integer must come before the larger one. In your case this doesn’t happen (-5 > -200), hence the error is seen. The interval b-a for math.random(a, b) can’t be negative. Just rearrange the numbers:

local velocityPlatform = game.Workspace.VelocityPlatform

function vel(z)
    velocityPlatform.Velocity = Vector3.new(0,0,z)
    wait(2)
end

while true do
    vel(math.random(-200, -5))
    -- You could also do vel(-math.random(5, 200))
end

local velocityPlatform = game.Workspace.VelocityPlatform

function vel(z)
    velocityPlatform.Velocity = Vector3.new(0,0,z)
    wait(2)
end

while true do
    vel(math.random(-200,-5))
end

You have to put the smaller number then the bigger number. It's a simple change, but can be unnoticeable. I'd prefer this instead :

-math.random(5,200)