SIM card not supported

I have had an iPhone 6 for several years before my mom bought a SIM card for it. I tried inserting the SIM card but the SIM card not supported keeps on popping up. I checked the carrier for my phone and the SIM card and they both fall under AT&T. My little sister's iPhone 6 (which was bought together with mine) activated immediately. I'm not sure if it's an unlocking issue or what else it could be.