SIM LOCKED

If it did not appear that your iPhone was SIM-locked when you bought it, then you must contact the place where it was purchased, they must inform you which Carrier it is locked to, and help you get it unlocked.

You can find more information here:

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

You will have to try to figure out who the carrier might be.

First, I would try.....Settings > General > About > Network or Carrier......to see if the info you need appears there.

Otherwise, I would check with the store. They should have that information in their records.

Only the carrier can unlock the phone, so you will need to get in touch with them. They may be under no obligation to unlock the phone.