Hello @wikipedia55 could you please tell us more information about the device you are using, the OS version and the supervisor version? Is the device online?
Thanks for the details. Are you able to reboot via the application container using curl i.e. curl -X POST --header "Content-Type:application/json" \ "$BALENA_SUPERVISOR_ADDRESS/v1/reboot?apikey=$BALENA_SUPERVISOR_API_KEY"
Also, with the 400 what is the exact error you receive? And is the --data field formatted as proper JSON?