There may be an issue with the Apple server as it is busy at times. Millions of Apple users connect and use apple services 24/7. So, there might be many other people using the service at the same time and you are seeing this trouble.
Your internet connection such as Wi-Fi or mobile network doesn’t work well.
The carrier that you are using might not allow you to activate your iPhone.
Maybe your iPhone is not recognized thus showing this issue.
There is a possibility that iTunes was unable to verify your iPhone.
Maybe the iPhone is still locked or linked to the previous user and the activation lock is restricting a suspicious activity.
The activation issue on iPhone 7 is most likely a baseband circuit fault on it. Apple stores offer free repair in this cas