Bro, same thing I experienced, though I unrooted the phone with superuser but it didn't take effect in twrp recovery. What u have to do is to download stockrom for tecno c8 which is 1gig, better u download it with system, then using sp flash tool to flash system.img.
Note:whenever you off ur phone, it will reboot back to twrp recovery, so u have to be vigilant when connecting the phone with flash tool using cord. Immediately u connect, hold down the volume - key till the task is completed. Or else it will keep booting as u are connecting the phone to d system. Faced lot of hell before I could get it. Then after completed and show download OK. Then u disconnect and on ur phone. It will take time to boot expecially in the tecno logo but be calm. Then after finally booted, then u can update your os. I hope this helps.
Sent from my TECNO-C8 using Hovatek mobile