The firmware is not installed on the RobinNano_v3.1 board.

[resevar]

Hello everyone (I apologize in advance if I write incorrectly, English is not my native language).
The mks_robin_nano_v3.1 board. I have flashed to Marlin_2.1 from a USB flash drive many times.
Recently, I wanted to install an inductive sensor, compiled the firmware into VSC and uploaded it to a USB flash drive. But the printer does not flash when turned on.
I tried different firmware, different flash drives, tried from an SD card and even turned on with the "reset" pressed.
All to no avail. At the same time, g-code sees and can print from flash drives and SD cards.

[hewasajumperboy]

I literally just went through this with a used board I acquired. Mine had a bootloader that wouldn't allow firmware updates. I had to use a Raspberry Pi and OpenOCD to flash it. I recently got an ST-Link v2 and it flashed the bootloader no issue. See my comment here on the procedure for OpenOCD on the Pi.