Only a few possibilities

Usually, when you hit the brake, the brake switch is triggered. That sends a signal to a solenoid that opens the shift interlock to allow shifting.

In your case, either the brake switch didn't trigger when you brake (which you might be able to tell from the brake lamp) or the solenoid didn't work (or didn't work effectively), or the connection between the two are broken.

If you're not used to fixing cars, take it to a shop. It shouldn't be very expensive to fix.