The taillight does not work. The light is intact, so either the lamp bulb is burnt out or the fuse is loose.
Although a lamp may look fine sometimes, a tiny break in the filament that is not quite visible can cause failure. The most accurate way to test a bulb is with a dvom/multimeter set to read ohms and measuring resistance across the bulb with it removed from the circuit. While a loose fuse is a possible cause for your non operative taillight, I would recommend a certified mechanic, such as the ones from YourMechanic, to inspect the power at the electrical connector to verify if power is making its way all the way back to the bulb. The outcome of the power test will tell you if its a problem with the circuit or just the bulb.