This is a scam.
1. Force Close Safari. https://support.apple.com/en-us/HT201330
2. Clear history and website data from Safari: https://support.apple.com/en-us/HT201265
3. Stop pop-up ads on Safari
Pop-ups and ads in Safari
If you see an annoying pop-up in Safari, you can go to the Search field and enter a new URL or search term to browse to a new site.
If you don’t see the Search field on your iPhone, iPad, or iPod touch, tap the top of the screen to make it appear.
Some popups and ads have fake buttons that resemble the close button, so use caution if you try to close
a pop-up or ad. If you're unsure, avoid interacting with the popup or ad and close the Safari window or tab.
Section: Close pop-up ads on your iPhone, iPad, or iPod touch https://support.apple.com/en-us/HT203987
4. Avoid scams: https://support.apple.com/en-us/HT204759
5. Identify legitimate emails from the App Store or iTunes Store