The bootloader in the Stylo 4 is locked, and not unlockable, so you can't get to it. (Fastboot is a program you run in Windows. The bootloader is what you go to int the phone to use fastboot on the bootloader.) Since it's locked, adb reboot-bootloader fails.
What don't you get? The fact that the thread was moved? You originally posted this in the Optimus G forum, which is for a very old phone, and has nothing to do with the LG Stylo 4. I moved your thread to the Stylo 4 forum, which is the appropriate forum since you're posting about that phone.