Try to use an absolute path like
/usr/bin/python3 /root/yahboom-hat/rgb-cooling-hat-display.py
After boot you can also use journalctl -u dietpi-autostart_custom
at any time to see the script output, if there is any.
Try to use an absolute path like
/usr/bin/python3 /root/yahboom-hat/rgb-cooling-hat-display.py
After boot you can also use journalctl -u dietpi-autostart_custom
at any time to see the script output, if there is any.