Firmware.bin on sd not flashing Ender 3 Pro running Marlin 2.1.2.1

Well, I guess what @EddyMI3D was trying to convey: Please follow the official instructions and not some video of unknown quality.

This already could be the issue. A lot boards struggle with SD cards >8GB

In Issues flashing the printer board you will find the common reasons that lead to failed attempts.