Why coolant light stays on

i would say its a sensor on or in the reservoir. this will not be on the plastic side where you fill, but further down the line. i would imagine on or near the radiator or perhaps past the emergency pop valve.

the sensor could either be malfunctioning, missing, or if it is inside of a compartment like the bottom of the radiator, it could be filled with debris and settled down covering the sensor giving it a false positive.

i only speak with experience from my own car issues in the past, so i will concede to any other diagnosis given here.