[Q] I can't restore back to stock please help! topic

So to clear things out my phone is not bricked it is currently rooted and has twrp recovery installed. I've been trying to restore it back to stock but the lg recovery tool will load my kdz file everything goes good then it tells me it can't detect my phone. I already installed and the usb drivers and still nothing I even tried enabling usb debugging mode and it still don't work. I also tried flashing in offline mode and it does the same thing it just wont detect my lg g2 can someone please help me out i want to send it in for warranty purposes but i need un-root it before i send it in thanks.