How to unblock carrier on my iPhone 8?

See the information in this support document. How to unlock your iPhone for use with a different carrier - Apple Support

You will need to know who the carrier is. They are the only one that can remove the carrier lock.

If you bought this phone used from someone, then return it. You will not be able to get the carrier to unblock it and Apple is not involved in carrier blocking at all. That would be just one of the many problems if you purchased the phone used and hopefully you do have a chance to return it as most of the time the sellers just disappear and the platform that was used will rarely help either. Before even consider purchasing a used device, review this:

The All Too Common SAD Reality of Buying … - Apple Community

If you did not purchase this device used, then you would know who the carrier is and as long as you are the customer, there is hope they will remove it if all the obligations have been met to facilitate the unlock.

You contact the carrier it is locked to and request an unlock.

If you are not a current or former customer of the carrier it is locked to, it is unlikely they will agree to unlock it.

Who is your carrier for the iPhone 8?

The carrier locked the phone

Only the carrier can unlock the phone

However......if you are not the customer associated with the carrier account.....the carrier is not going to help you because the phone has likely been reported as stolen.