Safari can’t find the server

Reset network settings. Have WiFi password ready before trying.

If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support

Turn off Private Relay and try.

About iCloud Private Relay - Apple Support

Turn on iCloud Private Relay on iPad - Apple Support

Some specific websites, or all websites?

Usual cause of this is a DNS issue or outage, either with your ISP, your Wi-Fi router, or with the website DNS, or possibly with the DNS support within the iPad. Or it can mean an invalid website address.

First try restarting your iPad. Also restart your Wi-Fi router. That can clear some issues.

If you’ve added a VPN app, remove it, restart, and try again.

Before a network reset, I’d check where your DNS server is set, and maybe switch that to manual DNS, and try 1.1.1.1 and 9.9.9.9 as the two DNS servers. That can bypass a problem with your Wi-Fi router, or with the ISP DNS.

You’re on iOS 9 per the footer, which means you’re going to have trouble reaching some websites anyway. Any sites using LetsEncrypt certificates for security will require manual steps to re-enable access. iOS 9 also means no private relay support, so that’s not a factor here.