It sounds like you are saying that you had not yet activated the iPhone. Unfortunately that means it is usable by whoever took it. Since you know who it is you can go file a police report but otherwise there isn’t much you can do. AppleCare won’t cover it (you would have had to have AppleCare+ Theft and Loss for it to cover this situation but since the phone wasn’t activated and connected to your Apple ID that would also mean Find My wasn’t activated which is a requirement for a claim to be made under AppleCare).