If it’s telling you the upload succeeded and it’s just failing to load the firmware, then maybe there’s an issue with the firmware itself?
Have you tried just flashing on a standard firmware version, both through an application (like QGroundControl), and using waf?
If no flashing is working (including known good firmwares) then there’s likely something wrong with some of your hardware, be that
- the USB port on your computer
- the cable you’re using
- the flight controller board
- note that the Pixhawk 2.4.8 is known to not be great quality
I’ve asked our main ArduSub developer if he has any additional thoughts, but it’s hard to provide insight on an issue like this without any kind of error messages or detailed information about the approaches you’ve tried.