Anyways, the answer to the question "how to enable sw decoding" is disable MMAL.
Pi1/Pi2 needs license for hw decoding Pi3 license is optional for hw decoding. Pi4 hw decoding not available.
MMAL means hw decoding. The option wording is misleading it says "allow MMAL" which makes it sound like the Pi will make a decision, it really means "use MMAL instead of sw".