You forgot to enable "OEM Unlock" in Developer Options before flashing. Fix: Reboot to stock ROM (you may need to re-flash stock firmware via Odin first), enable OEM Unlock, wait 7 days (Samsung’s RMM lock on older devices), then reflash TWRP.
Store this on your PC. If you brick the tablet, this file saves your life. twrp3021smt560tar full
In this guide, we’ll walk through how to flash the twrp-3.0.2-1-smt560.tar file using Odin. ⚠️ Pre-requisites & Warnings You forgot to enable "OEM Unlock" in Developer