The bootloader must be unlocked if you can obtain the token from lenovo support. There are ways to get around this by flashing a "bumped" kernel/recovery. But in your situation it might not even flash (locked). The bumped kernel might be designed so as to allow a custom ROM to pass verification. More on that subject here
Kernel/recovery from your device forum or from twrp website should already be bumped. The question is: will it flash?
ok, so what do you advice me to do first? I don't know what to do, it is the first time i go so deep in android and i have no knowledge / experience ... I feel a bit st^pid for all these questions but ...