TWRP only allows flashing into either the Boot or Recovery partitions

I'm trying to flash a Pixel Experience GSI onto my 2017 Samsung (SM-J730G) using TWRP . In order to do so, I need to flash the GSI's system.img into my device's System partition. However, my TWRP only allowed me to flash into either the Boot or Recovery partitions. The other options I see in other phones (System, Vendor, Preload, etc.) are nowhere to be found.