Flashing GApps from Recovery, getting Error Code: 20 When I try to flash GApps right after flashing LineageOS with TWRP I get,
Updater process ended with ERROR: 20
Error installing zip file '/sdcard/open_gapps-arm64-10.0-micro-20201114.zip

What is the cause of this problem, how can I fix it?
This is because you can't flash Open GApps right after flashing LineageOS. You must reboot back into the recovery TWRP, and flash the application after that.
From the Official Instruction, which support only LineageOS Recovery

(Optionally): If you want to install any additional add-ons, click Advanced, then Reboot to Recovery, then when your device reboots, click Apply Update, then Apply from ADB, then adb sideload filename.zip those packages in sequence.

But this is no different for TWRP. You can find people here in the bug reports for Open GApps that have the same problem.