But I'd start with going to Settings > Apps Search for the Browser & Click on it > Scroll down to Storage & Clear Both Cache & Data for the app & Reboot.
There's other things that you could do before the above suggestion, but without knowing more, it's difficult to give you other options.
As adviced by RedDogRabbit, clearing the browser’s cache and data is a solid first step, especially if the message is being triggered by a background ad or redirect. As mentioned, it would help to know:
What browser are you using? Did this start happening after installing a specific app or visiting a certain site? Does the popup appear even when you're not browsing?
Kindly share more details so we can help you further!
