Pumps needs to be energized then, pass the switches from off to auto. The transfer flag indicator it will pass to green if there is something to transfer...
If you start the mission with low fuel quantity, then only fuselage tank will contain fuel, and central wing and outer wing tanks will be empty and in red, so if you pass to auto, and they are in red, the pumps will be disconnected. In manual, pumps will be working even if there is no fuel to transfer, so you can damage them.