G3 custom roms wont load

ive followed several guides to do this via twrp I backup my original files etc, then I wipe the dalvik, data, cache,system via twrp then goto install, find my file on my SD card and install ...wait for it to do its thing then install Gapps then, reboot device and it sticks on the LG opening screen but ive noticed during install of the rom this comes up

unmounts of /system failed; no such volume

soooo ideas on a card please lol im stuck as I was flashing a new rom due to lack of wifi on recent lollipop update