Why it matters
A canonical to a broken URL tells Google to index nothing. This is a scoring gate.
How to fix it
Point the canonical at the live, 200 version of the page (usually the page itself).
References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
meta-description-lengthMeta description length outside 50–160 charactersmetadata-in-bodyCrawler receives <title>/canonical/description inside <body> (Next.js streaming metadata)canonical-missingNo single rel=canonical in <head>canonical-invalidCanonical URL is relative or contains a fragmentcanonical-conflictCanonical target redirects, is noindex, or canonicalises elsewherecanonical-not-selfCanonical points to a different URLhead-js-mutationJavaScript changes canonical, robots, title or description after loadh1-countPage doesn't have exactly one visible <h1> in the server HTML