Ensure your J2 Core has at least 60% battery life.
Put your J2 Core into : Power off, then hold Volume Down + Power + Home (or Volume Down + Volume Up while plugging in USB, depending on the specific regional variant). twrp samsung j2 core
Odin can only flash files when your phone is in a specific environment called Download Mode. Turn off your Samsung Galaxy J2 Core completely. Ensure your J2 Core has at least 60% battery life
: This means OEM Unlocking was not properly activated in Developer Options, or you are trying to flash a TWRP file built for a different model variant of the J2 Core. twrp samsung j2 core
Connect your official USB cable to your computer, but plug it into the phone yet.