Unable to mount /data after multiple wiped Everytime I try to flash anything from TWRP it says "E: unable to mount /data". I have tried multiple wipes and entirely reflashing the os but everytime i finish setting up the phone after wiping it, it goes back to that error.

Phone is a Samsung J5 Prime (G570Y)
How did you reflash the OS? using TWRP or ODIN?

A simpler solution is to flash Stock Firmware for your device via ODIN &amp; then flash TWRP again. 

You might first want to try the solution suggested on this XDA Thread if you are comfortable with using Terminal 
https://forum.xda-developers.com/showthread.php?t=2683787 

Reflashing the Stock Firmware via ODIN will most likely fix this issue.