READ THIS BEFORE FLASHING!
When you boot in TWRP-
* Go into Wipe-
* Go in Advanced wipe
* Select- Cache, Dalvik/ART cache, System, DATA
* Then flash the ROM & reboot to System!
If you get an error as mentioned in the video then boot back into TWRP recovery and backup everything that you need as it will wipe everything including internal storage of the DEVICE!
Once you backup necessary data, Go into Wipe- Format DATA, Type yes and format it!