Trying to unlock my new phone

Hey, so I was given a phone as a gift from my dad, and he bought it unlocked off of amazon, and when I went to put the SIM card in it (for metropcs), it gave me an error MM6, phone not allowed. I looked it up and all I can find is stuff about unlocking the phone, (a Samsung galaxy s7) so I followed the steps on AT&Ts website, and I got approved for an unlock code! But now I'm scared of messing up because I only get the 5 chances, and there isn't anything popping up for me to type the stuff into. I already looked at This that tells you where to go for the phone types, and also this similar question, but it didn't help, I still can't get it to work. Nothing pops up at start up, and when I try to use the dialer to enter it nothing happens, and if I try hitting call when I've entered it it just says 'not registered on network'. I'm not sure what to do, but I'm really frustrated as I've already had to send back a different phone of the same kind and reaaaaallllly don't want to have to try a third one.