Is AT&T can lock unlocked phone?

Where did you buy the phone and what told you it was unlocked?  If you bought from AT&T, it’s always sold locked.  If it’s bought from Best Buy or another place, it may have been sold under the US reseller flex policy where it’s technically unlocked and sold without a SIM card, but when you put a SIM card in, the software is designed to lock to the first SIM card installed.

I have brought this phone from Best buy. If  phone is locked on 1st sim then what is the use of buying unlocked phone. Also can you please let me know what can be done to unlock it again? Also i get to know this because once i changes the sim its showing me “please request service provider to unlock the device. This is not hardware issue. etc.”

It’s sold that way because that’s how Apple chooses to sell the those phones under their policy.  The online process to unlock the phone is here:

https://www.att.com/deviceunlock/unlockstep1

I have tried this method but i got mail saying ”your device Hasn’t been active for 6 months- try after 6month”

The six month message implies that you are using it with prepaid.  AT&T has a requirement for 6 months of continuous service before they will allow an unlock of a phone used on prepaid.  Right or wrong, that’s their rule.  If you bought it in March, you’ll have to wait a few more months.

That 6 month requirement for prepaid is intended for prepaid phones. An iPhone 11 is not a prepaid phone. It is possible to get an exception to the 6 month requirement but it is not easy. If you can prove paying full retail price for the iPhone 11, you may be able to get an exception. The best way to pursue is by filing an FCC complaint.

Yes, I have paid full retail price. Also if i can wait for few months then do i need to continue with current at&t service? means do i need to pay at&t service charges for remaining months? because currently i am out of country. @sandblaster thank you for your inputs. 

Not sure I understand your question. If you are able to get the exception, you don’t need to keep paying for service. If you just plan to wait the 6 months, yes, you must continue paying for service. The requirement is to have 6 months of continuous paid service.

Ok Got it, if i want to just wait for 6 month then do i have to do full prepaid recharge (currently using $75 pack) or just any small amount recharge is fine because any way i am not gonna use this card?

You must pay for whatever plan you have. Unfortunately, you have the most expensive one. If you could change to a pay per use plan, you would only need to pay $10 every 30 days or $25 every 90 days until you get to 6 months.