Activation Error

If you can't activate your iPhone - Apple Support - hhttps://support.apple.com/HT201407

Read the entire article, including about how to contact a specific Apple Support department at the very end.

This is indeed about a phone? You posted in the Watch forum.

Did you do as it instructs?

Restore your device from an iCloud or iTunes backup - https://support.apple.com/HT204184

Update your iPhone, iPad, or iPod touch - Apple Support