Send the command

A command is an opcode plus its arguments. Flight software answers in order: received, accepted or rejected, then executed. Acceptance checks the opcode and the argument range. Execution is what the telemetry shows afterward.

HEATER_ON is opcode 0x21. Its duration argument is a whole number of seconds from 1 to 60. Send it and read the log. A good command leaves HEATER_STATE at 1 and increments HEATER_CMD_CNT.