I bought a 2nd hand iPhone and it says the sim is locked

Unfortunately you will most likely need the assistance of the person who sold you the phone. Most carriers will not provide information about unlocking the device unless you are the account owner who subsidized the phone. They are under no obligation to unlock it for you and you have no idea if the conditions have been met by the original owner to have it unlocked. That is just one of the many problems associated with purchasing a used device that can be found here:

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

Best Option is Contact the Previous Owner If possible, get in touch

with the previous owner to ask if they can provide you with the SIM

unlock code or contact their carrier to unlock the SIM.

Or you can contact the carrier and request an

unlock. You may need to provide proof of purchase or ownership

information.

Ask the person who sold the phone to you. Go to settings general. About and see what it says there.