I suspect its an issue with magisk 20.0, as others have reported success rooting with my FW version (August security patch). How can I use a legacy version of magisk (V19.5)? The app seems to force an update even with WIFI off. A legacy ZIP doesn't help because we need to use recovery mode....
EDIT1: Found this custom update channel,
https://raw.githubusercontent.com/ianmacd/MagiskBuilds/master/updates/ianmacd.json
This provides V19.5 for now. Add it in your magisk settings
EDIT2:
No luck with Magisk 19.5, getting the same error....
I will try earlier versions...
EDIT3:
I wasn't able to get this method working, so instead I'm using the following kernel
https://xdaforums.com/tab-s5e/development/kernel-luiskernel-gts4lvwifi-gts4lvlte-t3977343
Follow the instructions outlined there to install TWRP, and then flash the custom kernel. This kernel allows for traditional magisk install method (flashing zip through TWRP). Expect a few bootloops before the OS kicks in (took me three boots, YMMV).
Works beautifully this way. Using Magisk V20.
Thanks to 8224Freak for the kernel