Hi Annban,
Welcome to Apple Support Communities. We understand a pop-up error appears while browsing the web on your Mac. We'll be happy to help.
Are you experiencing this behavior while using Safari or a third party web browser? If this is occurring with Safari, you have the ability to block pop-ups in Safari by following the steps outlined in this link: Block pop-ups in Safari on Mac
If that doesn't help, we suggest to restart your Mac in safe mode. This will perform a basic check of your startup disk, delete some system caches and more. Here's an article with additional details and how to go into safe mode: How to use safe mode on your Mac Then, you'll want to restart your Mac as normal and test to see if the pop-up continues to occur.
Please keep us posted with the results.
Kind regards.