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PUK code is the special key that you use to unlock the SIM card. It is not the lock code of iPad. Different SIM cards have different PUK codes. Only you and your carrier network should have access to it. Check the papers came with your SIM and you will see the PUK code on it.
Your SIM is currently blocked and you need the PUK code to unlock it. If you don't have the papers came with your SIM, card then you have to contact your carrier support to get the PUK code. Call your carrier network (AT&T, Verizon etc) and they will give you the code instantly after verifying some details.
Remember, DO NOT enter incorrect PUK codes. If you enter incorrect PUK mode multiple times, you SIM card will be PERMANENTLY disabled and you will have to get a new SIM card.
Only enter the correct PUK code. If you have any questions or would like any additional information, please don't hesitate to ask.