Mac Mini wont let me enter Apple ID to install OS X from app store

i got this mac mini from my dad, he said it was from his coworker who didnt need it anymore, try to open it and i need to install OS X El Capitan, try to install, need apple id for app store, put in id and password and it thinks for a bit before saying "An unknown error occurred." so i look on here, it says i might have an older model that doesnt have the whole 2 step code thing and that sounds right because it does send me a new code every time i try and enter my id, and then told to put in the 6 digit code after the password, i do that and the password is incorrect