why does low fuel symbol light up on 2013 juke tank is full

This sensor is getting the wrong signal, it itself may need checked for loose connection or whether it is subject and needs replaced, or the float in the tank has an issue, it may be stuck in the down position or float has came off the float arm allowing the float arm to move in the empty range. Watch and track mileage in-case it thinks you tank is full at the gas gauge but it could be actually empty or about empty unless you know you just filled it up, and if float has an issue the gas gauge will also reflect that too, so knowing when you filled it up can rule out the float as the issue