i have a bryant 80 heater flashing led code 12, what does that mean?

Bryant fault code 12 is BLOWER ON AFTER POWER UP which means that during the startup procedure on heat call from the thermostat it turned the blower on but could go any further because the flame sensor (or the circuit board that analyses its output) cannot verify the flames. It could be a for a number of different reasons which include the bad igniter, pressure switch, the flame sensor of the circuit board. Since you are saying the the gas will start flow but will not ignite, I would check the igniter. Is it glowing?

code 12 flashes, but the blower won't come on?