When trying to install non-official firmware, the screen shows "Firmware signature verification failed! Only official signed firmware can be flashed.".
Error name: Signature verification failed
Error code: #12606
The printer has detected the attempt to flash a firmware that does not have the private key signature that verifies the firmware as being official.
The Original Prusa MINI/+ in its factory state only accepts official firmware. In order to flash a custom firmware, you need to break the appendix on the Buddy board. See how to do that here.