My iPhone has carrier lock, how can I use SIM card in it

That is just one of the many problems when purchasing a used device. It is possible the seller has not met their obligations to remove the Carrier Lock on the device and there is no way for you to do that since the carrier will not give you any information about the account that is locking device.

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Best to return the phone for a refund, since you will not be able to remove the lock on the phone yourself.

If you want to try to use the phone, you will need to contact the seller and ask them to have the carrier remove the lock on the phone. The phone is useless until it is unlocked by the carrier.

You cannot do this yourself because you are not a customer of the carrier to which the phone is locked.