What does this error mean?

Oh, you're problem is that you're trying to index a table. You can't do that, you need to iterate through each object in the table instead. An example is here:

p=game.Workspace:GetChildren()
for i,v in pairs(p) do --Creates a loop that iterates through each value in the table.
	if v:IsA("BasePart") then
		v.CFrame = v.CFrame + Vector3.new(0, 0, 10)
	end
end

So there's an example. To apply this to your script, we will keep everything else the same, and fix up your loop:

function separate()
if game.Workspace:FindFirstChild("Stardrive")== nil then
    script.Parent.Sound.Pitch = 1
    script.Parent.Sound:play()
    wait(3)
    local s = game.ServerStorage.Stardrive:clone()
    s.Parent = game.Workspace
    game.Workspace.Separated.Value = true
    wait(0.5)
    local p = game.Workspace.Stardrive:GetChildren()
	repeat --Moved this outside of the 'for' loop. It would make each part move one at a time if not.
		wait(0.5) 
		for i,v in pairs(p) do
			v.CFrame = v.CFrame + Vector3.new(0, 0, 10) --Index's the value in the table
		end
	until  game.Workspace.Stardrive.Shape.Positon.Z <= 868.946
else
    if game.Workspace:FindFirstChild("Stardrive")~= nil then
        script.Parent.Sound.Pitch = -1
        script.Parent.Sound:Play()
        game.Workspace.Separated.Value = false
        game.Workspace.Stardrive:destroy()
    end
end
end

script.Parent.ClickDetector.MouseClick:connect(separate)

Ok, so now your code should work correctly. If you have any further problems/questions, please leave a comment below. Hope I helped :P

####-Dyler3