Your iPhone could not be activated because the activation server cannot be reached. Try connecting your iPhone to iTunes to activate it, or try again in tew minutes

This is a user-to-user forum. We can’t answer “why” questions, because we don’t know who you are, where you are, what device you have, and how you are trying to activate it. To do some troubleshooting here is an Apple support article—>If you can't activate your iPhone - Apple Support

And here are some additional notes:

• It really is unavailable. Rare, but it happens. You can check its status here: https://www.apple.com/support/systemstatus/
• The phone does not have a valid SIM installed.
• You are connected to a VPN.
• Your antivirus or firewall is blocking access to gs.apple.com if you are trying to activate using your computer. Try temporarily disabling it.
• Your router's firewall is blocking access to gs.apple.com or to a port that it requires (443).
• Your version of iTunes is out of date if you are activating with your computer. 
• Your phone is jailbroken, or it was at some time in the past.
• Your computer was used to hack or jailbreak some iOS device in the past (not necessarily the device with the current problem), and its network database was corrupted. Hacking includes using some questionable apps such as recboot, reiboot, Tinyumbrella, redsn0w, etc. If this is a possibility inspect your computer's hosts file. If there are any lines in the file that reference gs.apple.com that is the explanation. You will need to rebuild the hosts file.
• Your phone has a hardware problem. Take it to an Apple store or contact support (link at the top of this page) for diagnosis.