Are you trying to flash ntrboothax onto the R4 Gold Pro with a 3DS that does not load DS software? That will not work (
ntrboot should work as soon as the flashing is done with another DS). This is why the
R4i Gold 3DS Plus with the internal switch for ntrboot/normal operation would have been a better choice for you.
However, if you find any other DS system – no matter which model as long as it loads DS games – you can flash ntrboot with that:
From the guide, Italian version:
Prosegui con il
Flash di ntrboot (Con più 3DS)
Prosegui con il
Flash di ntrboot (NDS)
Prosegui con il
Flash di ntrboot (DSi)