How can I unblock my iPhone's SIM card using PUK code?

Contact your carrier for assistance with this. Apple is not involved.

You’ll need to contact your carrier. You cannot do it on your iPhone. For example AT&T’s instructions tell you to login to your AT&T account, select your device, and click the PUK option.

Unblocking is a purely carrier function that Apple has no control over. Follow your carrier’s instructions.

By contacting your cellular carrier and asking them for your SIM's PUK code.

Regards

Giulio