Trouble with updating my system - Huawei p8 Lite
TWRP is a custom recovery. You have to do these things before uphrading 


Uninstall TWRP ( after taking a backup)
Install stock recovery
Unroot
lock bootloader
Upgrade using the key combination method, which wipes data and does a clean install- always preferred for a version upgrade
Unlock bootloader and root


This XDA post does not talk about rooted devices but this mentions that out will not work on rooted devices. Procedure is from my experience with Honor 6