double filament bulb installed incorrectly defective socket bad ground
Have you tried changing the bulb ? maybe the bulb is bad
It could be a bad bulb, it could be bad wiring, it could be a bad switch.
You have a bad ground on that brake light bulb.
Perhaps the bulb is bad?
Check for bad grounding of light socket or the wrong light bulb
Blown bulb/ bad bulb
Try checking the bulb, the brake light bulb may be blown.
You have a wiring issue or a bad brake light bulb.
Why not just remove the bad bulb and look see what model it is?
The brake light switch is up underneath near/connected to your brake pedal arm. Also, don't rule out that you simply have a bad bulb or corroded connection possibly causing the issue. If it is a bad bulb, see sources and related links below for bulb information.
Blown fuse, bulb, or bad brake light switch.