Bought iPhone XR off marketplace carrier locked

You either return the phone for refund or you contact the carrier and find out just how much it will cost you to haven them unlock the phone. Clearly whoever you bought it from either lied or scammed you. Apple cannot unlock the phone.

Facebook marketplace is one of the absolute worst places you can purchase a used iPhone from.

Did the phone open to the Hello Page when you turned it on? If it didn't, it wasn't prepared properly for a new user and if it didn't, you're going to likely have issues like Activation Lock to deal with, which you cannot bypass.

Read this --> https://discussions.apple.com/docs/DOC-250001038

Unfortunately Apple will not be able to help as a Carrier Lock is applied by the carrier and the only resolution is to contact them to see if the financial obligations have been met for the device. This is sometimes difficult as the carrier is not obliged to unlock the device for you as the contract is with the original owner. In some cases they won't even be able to provide any information about the device since the contract is not with you and they have no incentive to unlock it so you can take it to a different carrier.

Hey, try to contact the carrier and ask if they could unblock it but just remember they are not obliged to. In the future buy of the official apple market. Also I suggest contacting the official apple support.

This is not an Apple issue at all.

The fact that the phone might have been paid off does not necessarily mean that the carrier has unlocked the phone. Did you find out which carrier the lady was using on her phone?

If the lady was using the same carrier as you are, things don't add up. Please provide more details.