How to unlock sim card carrier???

That's correct. That is because it is locked to a specific carrier, which is not the one you are trying to use.

ONLY the carrier it is locked to can unlock it. The carriers in Japan do not unlock phones. Your phone can NOT be unlocked.

You ask the carrier to remove their sim lock.

They are saying that you need to ask the carrier to do it. They are the only ones that can.

You can’t. Japanese carriers will not unlock phones.

You can't.

Return it to wherever you got it and get your money back if you can.