Trying to buy iPhone but...

This is the message I get when I'm trying to buy a new iPhone (not for myself but for a relative):

Your current AT&T account type isn't supported on apple.com.

If you'd like to proceed, you'll need to purchase a full-priced iPhone, and insert an active SIM card or contact AT&T to activate your device.

I understand I can still purchase the new iPhone, but I want to know what this means before proceeding, especially because it's supposed to be a gift. What does it mean to have to "insert an active SIM card" to activate the device? I don't know if this matters but the intended recipient currently has an old iPhone (I think iPhone 6). Is this the reason for this message? Thank you for your help.