Which TWRP did you use? You need one which works with the Nougat stock rom.
I used the latest one from here. Not ashyx's one.
https://dl.twrp.me/gts210vewifi/
I'll post the steps I used again
- Flash stock nougat rom using Odin.
- Boot into stock rom and ensure oem unlocking is enabled in developer settings.
- Turn off tablet and go into download mode.
- Flash the twrp tar file using Odin.
- Immediately manually boot into TWRP after flashing in Odin. Instructions are in ashyx's thread.
- Go into wipe, and then format data. Type "yes" to do it.
- Reboot into TWRP again.
- Flash supersu zip and then the no verity no encrypt zip. I flashed from the external sdcard.
- Reboot
- Supersu should now appear among the apps.