Also, on older models the bulb socket contact points get a little corroded and it changes the resistance. That's enough to trigger the error. The bulb will test OK but it's the socket. The sockets twist into place. Sometimes twisting all the way then rotating back slightly will provide a new contact point and all will be well.
You can do some tests to try to isolate the problem light. If the error appears when pressing the brake pedal it's one of the brake lights but won't go off until the engine is turned off. If it comes on when using the turn signals it's one of those. If it comes on when switching on the low beams... okay you get the idea.