Unlocking Carrier-Locked iPhone Sim

You will need to Contact your carrier, there is nothing any of use or Apple can do. Here is an Apple Support article that details the issue, but all roads lead to contacting the cell provider.

How to unlock your iPhone for use with a different carrier - Apple Support

Only the carrier to which the phone is locked can unlock the phone.

Apple cannot unlock the phone.

The carrier will not likely unlock the phone unless you are a current customer in good standing.

Contact your carrier and ask them to unlock the phone.

If you bought the phone from someone, return the phone for a refund. You don't want that phone.

Only the carrier that locked the device, can remove the lock. Do you know the carrier that locked it? Do you know why it was locked?