When trying to install non-official firmware on your Original Prusa MK4/MK3.9, the screen shows "Firmware signature verification failed! Only official signed firmware can be flashed".
Error name: Signature verification failed
Error code: #13606 (MK4) #21606 (MK3.9)
The error is issued when your printer has detected an attempt to flash firmware that does not have the private key signature that verifies the firmware as being official.
The Original Prusa MK4/MK3.9 in its factory state only accepts official firmware. In order to flash a custom firmware, you need to break the appendix on the xBuddy board.