When pressing the brake pedal a light appears on the dash board with an X on it on 2007 Hyundai Elantra

I check the break light all are working.. It happen all the time when I step on the break pedal

Asked by for the 2007 Hyundai Elantra
2 answers
See owners manual.
poss 3rd brk light has 2 bulbs and one is bad.remove and ck