for your phone to have bixby voice your phone must be running on nougat. till now, as far as i know bixby voice is only available on s8, s8+, note 8 etc. which are newly launched phones.
sorry for offending you bro but i dont think your on8 will get bixby voice update. and for those who ask for software release from 5.1.1 to 6.0. well for going on to 6.0 your phone needs to be built on x64 architecture. as far as i can remember, almost all phones which are built on x64 architecture have already got 6.0 updates. it has nothing to do with software problem. its the hardware fault which has become too much old.