S6 edge Sm-G925a unlock problem

why dear sir i can’t dricet unlock my SM-G925a version 5.1.1

i try many times but it’s need root must i found many sites but not success :'(

i invest on my box today for this cell phone but results 0 …

please i need solution for this phone .. its american At&t model …

thanks … waiting your good response … be blessed