What does attempt to index nil with 'Position' mean?

MouseButton1Click does not have a parameter called hit that returns the player's character. You need to access the player's character by using game.Players.LocalPlayer.CharacterAdded: Wait(). Here's an example:

local char = game.Players.LocalPlayer.CharacterAdded:Wait()


script.Parent.MouseButton1Click:Connect(function()
    char.Position = Vector3.new(-18,0.5,-9)
end)

This is only for use in a local script, which will not change anything for the server or the other players.

This is what you use for a normal (server) script:

game.Players.PlayerAdded:Connect(function(player)
    player.CharacterAdded:Connect(function(character)
        script.Parent.MouseButton1Click:Connect(function()
            character.HumanoidRootPart.CFrame=CFrame.new(Vector3.new(-18,0.5,-9)
        end)
    end)
end)

This is what you are looking for. Am I correct?