[G Pad 8.3 General] roirraW "edor" ehT:
My ViKtory810 ROM already includes and flashes the right bootloader, which is 5.0.2, either the 35A or 36B aboot.img depending on which version of the ROM you installed. The only way you should be getting the secure boot error is if you had changed your bootloader version - although maybe it gives that same message when trying to get to TWRP and you still have the newer bootloader installed, I don't remember. Normally, you would re-downgrade the bootloader to 4.2.2 in order to, for example, get into TWRP - which at the very bottom of my ROM OP I note that you have to do just that to get into TWRP, and then from TWRP, once you're finished doing whatever you need to do in there, you have would have to reflash the proper bootloader (the 35A one if you have 35A ViKtoryROM installed, or the 36B one if you have 36B installed).

In my tests I've found that official 35A and 36B LG Lollipop for this device won't boot without the properly matched bootloader.

Could you clarify for me which things you can or can't get to? Can you boot the ROM? Can you get to TWRP? If you can boot the ROM and can't get to TWRP, that is because, as you said, you didn't flash SuperSU.zip from TWRP before booting ViKtoryROM, which is necessary in order to be able to re-downgrade the bootloader later in order to get back into TWRP when you want.

Can you get into Download mode? If you can get into Download mode, I've never come across an instance where using LG Flash Tool 2014 to flash a KDZ cared what bootloader, recovery or anything was installed.

First, install the drivers I linked to two posts back. Then reboot your PC. This isn't always necessary but it doesn't hurt anything and I've found that solved some driver installation problems for myself occasionally.

To get into Download mode, turn the tablet completely off. Wait for it to vibrate slightly as the screen goes black for several seconds before it actually fully shuts off, and it vibrates when it does so. Then, while holding the Up Volume button and having the USB cable plugged into your PC, plug the MicroUSB end of the cable into the tablet. Keep holding the Up Volume button until you see the faint "Download Mode" in the middle.

On your PC, open Device Manager and make sure you see the tablet listed under "Modems". If it's listed there then you're good. If it's the first time you've installed this particular driver version and first time you're plugging the tablet into the PC while in Download mode on the particular USB port you chose, it can take all the way up to 10 or 20 minutes to load the driver. That's extreme and again, doesn't always take that long, but it should be relatively obvious in Device Manager when it's fully loaded.

Once it is, then use LG Flash Tool 2014 to flash the 24B KDZ (or any KDZ 24B or older), then use the Stump apk in brute force mode to re-root the device, downgrade the bootloader to 4.2.2, install TWRP 2.7.0.1, upgrade TWRP to 2.8.7.0, then reflash ViKtoryROM810 36B and this time also flash the SuperSU.zip I link to in the ROM install instructions since once you reboot out of TWRP, you won't be able to get back into TWRP unless you use root from the running ROM to temporarily re-downgrade the bootloader.