The checkbox for visual notifications will be disabled if you have notifications blocked for libraryh3lp.com in your browser.

 

To unblock, click the lock icon in the URL bar and update "Notifications" to "Allow" or "Ask" and then reload the webclient page. If you set the permissions to "Ask" instead of "Allow", you'll need to go back into the webclient preferences page and click the "Show visual notifications?" button to enable notifications.

 

Example screenshot from Chrome: