HapticService's Vibration doesn't work Online?

There is a small delay when roblox starts to when the controller is connected so you need to wait for the controller to be connected you can use the events GamepadConnected and GamepadDisconnected which fires when a controller is connected and disconnected.

Example:-

-- services
local userInpServ = game:GetService('UserInputService')
local hptServ = game:GetService('HapticService')

-- wait for game pad
print('Waiting for gamepad')
local gamePad = userInpServ.GamepadConnected:Wait() -- wait for a controller
print('Gamepad found', gamePad)

-- motor
if hptServ:IsMotorSupported(gamePad, Enum.VibrationMotor.Large) then
	hptServ:SetMotor(gamePad, Enum.VibrationMotor.Large, 0.6)
	wait(0.3)
	hptServ:SetMotor(gamePad, Enum.VibrationMotor.Large, 0)
else
	print('Motor not supported')	
end

I hope this helps.