Consider the shown simplified diagram. All parts connected to the negative terminal of the battery (0 Volt) are shown in blue, all the components on a positive voltage (>0 Volt) are shown in red.
When the potentiometer P1 is fully turned clockwise, there is no voltage at the input (Uin = 0 V)...
At the output is no voltage (Uout = 0 V). The LED remains dark.
What happens when you increase the input voltage Uin using the potentiometer...?