How do I get a mck code

That would probably indicate that the phone has been permanently locked to AT&T because someone entered an incorrect unlock code multiple times.

AT&T does not have the MCK code to give you, they only have the unlock code. They can however get the MCK code on your behalf from the manufacturer.

If you have or can get a proof of purchase, you can pursue getting the unlock code and MCK code by filing a better Business bureau complaint against AT&T. (If you purchase the phone from AT&T It shouldn't be a problem)

If you do not have proof of purchase, are not the original purchaser of the phone, but picked it up secondhand or something, AT&T cannot help you