If I buy iPhone 12 Pro from America, can I unlock the SIM?

If you buy a phone from AT&T, it will be locked to AT&T, and not usable on other networks until it is unlocked.  In order to be eligible for unlocking, the phone has to be paid off, and have completed a minimum term of active service with AT&T.

My suggestion is go and read the new apple locking policy.

@GLIMMERMAN76  perhaps you could provide a link or synopsis of what you are talking about?

I will once the apple site comes back up.  The apple website states phones are unlocked unless they are bought on payment plans thru the carriers.

Notice that I specifically said phones bought from AT&T.  I didn’t say anything about phones bought or financed through Apple, or anyone else.

I paid cash for the wife's 12 pro and it's att and it's unlocked.

Did you buy the phone FROM AT&T?  That’s different than buying an AT&T phone FROM Apple and either paying full price or financing through Apple.

Or are you saying that you can now walk into an AT&T store and buy an unlocked phone?

Her phone came from bb.  In the past if you insert a att sim card I would have to unlock it because of the us flex reseller policy.  Her 12pro did not auto lock.  Since apples activation servers handle locks something has changed.