Help bootloader unlock allowed (no)

Hy friends iam new user at sony xperia device i buy a old xperia z (c6603) when i returned at home i do some search about flash custum rom and openning the bootloader when i tap a code in dealer *#.... I show the statut (bootloader unlock allowed (no)...
My question is can i open the bootloader..can i install custom rom with locked bootloader and root my device :(