How to eliminate Popup ads

The first step is to check just one Safari setting is configured:

Settings > Apps > Safari > [General] Block Pop-Ups - set to ON

Next, unwanted content - such as pop-up Ads - can largely mitigated by installing a good Content and Ad-blocking product. One of the very best and most respected within the Apple App Store - designed specifically for iPad, iPhone and Mac - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable. All processing takes place on your device - and contrary to potential expectations, Safari will run faster and more efficiently. 

Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content blocked form download. A further benefit on metered services, such as cellular connections where you data may be capped or chargeable, this not only improves speed but also saves you money. 1Blocker has also introduced its “Firewall” functions - that are explicitly designed to block “trackers”. Being implemented at the network-layer, this additional protection works across all Apps.

Since Safari nor Apple's native operating system has any virus popups or ads of any kind, this would indicate the popup is coming from the webpage you are visiting. Also your device cannot get a virus and the message is simply a phishing one which typically comes from insecure websites that are poorly secured or host malicious ads.

Most likely it is not a pop-up; it is a web page designed to look like a pop-up: Dealing with Safari popups that won't go … - Apple Community

Scam Pop-ups, Scam Viruses, Scam Receipts… - Apple Community

Also: Block pop-up ads and windows in Safari - Apple Support