Need puk for unlocked at&t phone to use t mobile sim in it. Phone eeds to unblock

Puk code is on your online account. Prepaid you must call for a puk code

If you inserted a T-Mobile SIM card into an unlocked AT&T phone and are being asked for a PUK code you're not going to get it from AT&T you're going to get it from T-Mobile. A PUK code locks a particular SIM card to a particular phone so that no one can take your sim card and use your service.  For example let's say someone takes your phone but it's screen locked and you use find my iPhone to lock the phone so the phone is useless but they can still pop out the SIM card and use it in their phone, unless you puk-locked the SIM card. So in other words if you're getting a PUK request it's for the SIM card which you have indicated is T-Mobile's SIM card.

Ask T-Mobile.

A Sim not supported message would mean that the phone was still carrier-locked to AT&T and that request is done online.

https://www.att.com/deviceunlock/