UPDATE

USB key wasnt formatted as FAT32. Not sure why that didnt click in my head. Working now.

Raspberry Pi 3B
Lineage OS / Android 9

OS is installed
Reboot into TWRP
Go to select USB drive to install GApps
USB-OTG shows 0 for space

Go into Advanced > Mount
Unable to mount USB-OTG

Has anyone run into this?

Ive tried it with a USB key as well as an SD card reader/adapter