Why is my data saying error?

See this support article:--> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart - Apple Support

If the issue persists, contact Apple support, using the link provided in the last line of the above article.

Regards

GIulio

Try the steps in this article:

If you see No Service or Searching on your iPhone or iPad - Apple Support

If it still does not work:

Contact Apple for support and service - Apple Support

Because that means there might be something with your iPhone.

Good luck!

try this: https://support.apple.com/en-us/HT201415

Ask your carrier for help.