Why it matters
Google renders pages with their scripts and styles; blocking /_next/ makes it see a broken page.
How to fix it
Never disallow /_next/ or other asset paths; disallow only private routes such as /api/ and /account/.
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.
robots-txt-unreachablerobots.txt returns 5xx/429 or times outrobots-txt-missingNo robots.txtrobots-txt-invalidrobots.txt serves HTML, exceeds 500 KiB, or has unparseable linesrobots-blocks-allrobots.txt disallows the site for * / Googlebot / Bingbotrobots-sitemap-directiverobots.txt has no absolute Sitemap: linesitemap-missingNo reachable sitemapsitemap-invalidSitemap isn't valid sitemap XML or exceeds protocol limitssitemap-url-invalidSitemap <loc> is relative or on another host