Furnace constantly runs no flame

Could be many different things,
The easiest to do would be clean the flame sensor( a small metal rod next to the pilot opening, with a wire leading to the valve) some fine sandpaper will do.
Could be the hot surface ignitor


How old is the furnace?
It could be the thermocouple if it is an older model

Could be the gas valve

Could be the circuit board a lot of possibilities ,

I'd say clean the flame sensor, if that's not it, call a service tech

Most of the time if it is just the fan on it will either be a stuck limit switch or or pressure switch did not close