what would happen if I try to flash a custom recovery on a locked device? topic

I have bought a used TF300T. I want to install on it a custom ROM.
I can't unlock using the Asus tool (version 8 on system 10.6.1.15.3), probably because the serial of the tablet is already registered to the previous owner.
If I go on with the guide for making unbrickable the tf300t on AndroidRoot dot Mobi
what would happen?