tried to factory reset lg g pad while using twrp v2.8.6.0 system partition wont mount

so i was gonna try to sell my lg g pad 8.3 and so i went to factory reset it through the settings menu. the thing booted to twrp and so i picked factory reset. when i went to boot up after, it said "Secure booting Error! Cause: Device UnLock!, so Boot Success!!" and then boots to twrp. when i try to install a custom rom it says this: see attatched pic
and then just says the secure booting error message and just boots into recovery... any help with fixing this would be greatly appreciated. ps: oh and for some reason it will reboot randomly when performing various operations in recovery...

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