The RUU has to be flashed in fastboot using the fastboot commands, so if you're trying to flash it in TWRP it will fail. To flash the RUU, you have to:
fastboot oem rebootRUU
fastboot flash zip <nameofzip>.zip
First time WILL fail, so then run the same command again - fastboot flash zip <nameofzip>.zip
fastboot reboot
This is ONLY for the RUU ZIP file and not a normal TWRP-flashable ZIP
I put a ROM in my sd card and managed to flash it even thought it failed, but now i got a system going on. Now i can access fastboot!!! I used your method and the RUU u gave is actually what i used!!! And it worked!!!!!!!!!! Thank you so much man!!!! I dont even know what to say!!!!!