How to unlock iPhone 8 SIM Card?

If your SIM card is locked, you need to contact your carrier for instructions to unlock it. They may give you a PUK code, or they may tell you to go obtain a new SIM card.

If your iPhone is locked to a carrier and you want to unlock it, you again need to contact your carrier. Only the carrier can remove their lock. See How to unlock your iPhone for use with a different carrier - Apple Support

You will need to contact your cellular provider. After you enter the wrong PIN code for the SIM it will ask for your PUK code. If you enter this code incorrectly too many times it will destroy you SIM card and you will be required to purchase another one. Only your cellular provider has your SIM code and PUK code, so that will be your only solution. Apple does not have this code and you will not be able to find it yourself.

Contact your carrier for a PUK code.