on 5.1.1 u must use custom kernel to root, heres a link for it
https://drive.google.com/file/d/0B9u...ew?usp=sharing
do adb shell then
su -c am broadcast -a android.provider.Telephony.SECRET_CODE -d android_secret_code://IOTHIDDENMENU
Device Detected...
model:= SM-N915P
Androids Version:= 5.1.1
Description Version:= tbltespr-user 5.1.1 LMY47X N915PVPS4COK2 release-keys
Platform:= apq8084 [ qcom ] [ mdm2 ]
Baseband:= N915PVPS4COK2
VER:= N915PVPS4COK2/N915PSPT4COK2/N915PVPS4COK2/N915PVPS4COK2
Carrier := SPR_USA
EFS CSC := SPR
AP UART Status := Disable.
Network Mode := 8
Simstatus:= ABSENT
IMEI-1 = null or empty
ANDROID-ID = 1599da9d_0000E5310C
Device is not rooted
SAMSUNG ALL SPRINT UNLOCKING STARTED...
Checking Data...
Waiting for Loader...
Loader Ok...
Direct Unlock Success...