NucleoF446RE: Bootloader stuck on “waiting for the bootloader…”

I have uploaded the boot loader px4fmuv4_bl.elf on to Nucleo F446RE board at 0x0800000. The upload was complete without any errors. Next, I tried to upload the px4 firmware and then I get “waiting for boot loader”. I also tried with black magic probe to flash the boot loader following these instructions. Can someone help me with this.

I think it’s because that board does not support USB.
The USB connector is for the JLINK debugger and does not go to the processor chip.
Could be some clever work around, but on the surface it look like a nogo at least for the bootloader/
You may be able to flash the PX4 code directly on to the board.