Since the w3c Validator switched to the 'nu' engine, it hasn't been possible to support validation of all pages of a site.
Users with the w3c validator installed locally who find that it still works with Scrutiny, should continue to use Scrutiny v5.9.18 which is the latest version that can validate all pages.
Otherwise, you can still validate a single page at a time, use the context menu on the SEO results table.