- Restart the device. That's it. If issue is not resolved... Force restart iPhone - Apple Support
- You may not have internet, for your Internet connection.
- Over Cellular: contact your Carrier (Cellular Service Provider). The agency who issued the SIM to you
- Over WiFI: Contact your ISP (Internet Service Provider), the agency who provided the Internet Service on your Modem/Router to you
Please look at this --> If you can't activate your iPhone - Apple Support
If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support
If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their apple ID