Hello JennMartin,
Thank you for using Apple Support Communities! I understand that you're encountering a recurring pop-up in Safari on your MacBook Air, which directs to a website with the message "The page you're looking for can't be found." You've come to the right place for help.
The following support article provides suggestions to help block pop-up ads on your Mac:
How to block pop-ups in Safari - Apple Support
You may also want to try clearing your browser history in Safari on the Mac, then test if the pop-up window returns:
Clear your browsing history in Safari on Mac - Apple Support
If you're still encountering the pop-up, does it appear after a specific action such as clicking a link, opening Safari, or something else?
Cheers!