what does it mean when your iphone 8 says harmful viruses from adult sites and it says to download vpn app. what do i do?

Any such message will be a scam.

https://support.apple.com/en-us/HT204759

It’s a load of garbage trying to convince you to install their useless app. What the heck is “28.1% damage“ from a virus anyway - it’s not even an intelligent fear mongering message!

The lows that some companies will go to just to push their app is just sad.

What's really sad is how many people likely believe this load of b-crap and spend hard earned dollars for nothing.