Safari
Safari will not open web pages. I've got more than one ”reason” but the most often is something about the address being incorrect or not existing.
iPhone 7 Plus, iOS 12
Are you typing in the correct web site address?
Try double clicking the Home button and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.
Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch
Safari website data on your iPhone or iPad - How to clear
Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch
Try a forced restart.