i have an unlocked code

The ONLY way to enter the unlock code, it to put in a foreign SIM so that it will ask for the unlock code.

thank you for your response to that.....okey ,i read a comment.. Without a non- AT&T sim, enter ##7820# in your dial pad.....put in the unlock code ...any idea what to do next ? i try to press enter nothing happened....Did anyone know.....

This is primarily an unlock keypad sequence for Lumia phones, not sure what brand/model you have?
You can likely figure out if there's something similar for your phone (if it's not a Lumia), through a somewhat simple, but targeted search.

If you're getting that screen, the one with the padlock on it though, when using this sequence, you're likely on a Lumia device, or using a phone that mimics this for their firmware.

On the Lumias, when you enter the (15 digit I think) code, the paclock picture changes, from a locked padlock, to an un-locked one. 

The un-locked padlock symbol indicates you were successful.

That's how it's been on the 4-5 Lumia devices I've unlocked this way, I've never actually used another carrier's SIM to do an unlock.  I beleive the SIM is the guaranteed way to initiate the sequence though.

Thank you all for your quicked response...and my problem for unlocking my LUMIA 640 was being solve...

Thank you so much....