Can someone please help me with this?
Whilst rocksteady makes a valid point to post in the Avast One sub-forum, it may not necessarily differ from potential advice given and or questions asked.
When you get an alert, click the See details option as that could give us more information to work with.
However, I assume that you aren't trying to reach this website in the image ?
If not, first clear your browser (which one ?) cache and cookies and restart your browser.
If this resolves it you should be good to go:
If not have you installed or updated any add-ons/extensions recently or updated ?
Try running your browser with add-ons/extensions disabled.
If that resolves it then it is likely to be an add-ons/extension, now finding which one is going to be a slow process.