How can I activate my iPhone using iTunes?

Please see If you can't activate your iPhone - Apple Support

If you still can't activate, here's how to activate your iPhone through iTunes instead:

1. Make sure that you have the latest version of iTunes.
2. Check that your computer has an Internet connection.
3. Open iTunes on your computer. With the cable that came with your iPhone, connect your iPhone to your computer.
4. Wait while iTunes detects and activates your iPhone:
• If iTunes shows, "Set up as New" or "Restore from Backup," it means that iTunes activated your iPhone.
• If an error in iTunes says that the SIM card isn't compatible, contact your carrier.

Assuming your activating a SIM card.

see >> https://support.apple.com/en-us/HT201407