<html> element does not have a valid value for its [lang] attribute
You'll get this error if you have added an incorrect language code in the Project Settings.
Google Lighthouse says
To ensure correct pronunciation, screen readers use a different sound library for each language they support. Screen readers can switch between these language libraries easily, but only if a web page specifies which language to read for a given piece of content.To ensure correct pronunciation of the page as a whole, you must specify a valid BCP 47 language for the element.