today I try to upload the stable firmware for my NanoVNA 2plus4 after putting it on DUFU mode I get a white screen, opened Nanowiu (QT) I get a USB error that did not recognise the USB device, I reinstalled the USB driver but no work the screen stuck on white

I got exactly the same problem some months ago. It has been sitting there collecting dust.

To unbrick your device, use the steps here to enter bootload mode and flash firmware version *20220814* :
https://nanorfe.com/nanovna-versions.html

Although it has been told many times by developers that the bootloader mode will not fail during firmware upgrade, my device can no longer enter bootloader mode as the PC does not recognize it. The failed device had the backlight turned on as if under bootloader mode but the virtual COM port did not show up and PC reported an unknown device.

In a few occasions it seemed to enter bootloader mode (virtual COM port showed up) after numerous button press + power cycling, NanoVNA-QT still could not update the firmware. The QT program just hung up and did nothing.
Similar bootloader mode failures had been reported by other non-legitimate device owners. My device was purchased from Tindie but since the v2plus4 bootloader is closed-source and I found no other references, I ended up accepting the fault was caused by my mis-operation. I shall be grateful if there is a way to revive my device.

Hi, this may be an issue with Windows not detecting when a plugged in USB device is power cycled. Please try unplugging USB, holding down the JOG LEFT button, and plugging in USB, or if a battery is inserted, enter DFU mode first before plugging in to USB.

If your com port nummer is high. Try to reset to com port to a lower value e.a. 3
The firmware cannot handle high com port values
Go to device manager and follow as shown in the picture
Select your vna com port e.a. 15  and change the comport for e.a to 3.
Continue even when the com port is in use.

Thanks for all the advice but no luck. I am unable to enter bootloader mode after an hour.
I got another genuine 2.8" device which can be flashed with com57 though.

I actually had not started to flash anything when the device bricked. I was wondering if there was some other issues causing it not to run even under normal mode.

C.M. - have You tried NanoVNA-APP for flashing FW? Have You installed Cypress Driver?
I use this program for flashing NanoVNA V2 (SAA-2) and NanoVNA-H4 (this program works with .bin, .dfu and .hex files)
Best regards
Vladimir, dl7pga

Thanks, I also had problems with the new firmware. It was successful the first time with this SW.