Make sure you're holding the "G0" or "BOOT" button while plugging in the USB or clicking "Burn"—this puts the Cardputer into flash mode. Also try a different USB cable (some are charge-only), and plug directly into your PC (avoid hubs). Lastly, check Device Manager to confirm the COM port is stable and not switching. If all else fails, try flashing with esptool manually via command line.