Why it matters
Google shows the favicon next to search results and needs a crawlable, square (≥ 48×48) ICO/PNG/GIF/JPEG/BMP. SVG-only favicons aren't in Google's supported list.
How to fix it
Add app/favicon.ico and app/icon.png (keep icon.svg as an extra if you like).
Example
app/favicon.ico → <link rel="icon" href="/favicon.ico" sizes="48x48">
app/icon.png → <link rel="icon" href="/icon.png" type="image/png" sizes="512x512">
app/apple-icon.png → <link rel="apple-touch-icon" href="/apple-icon.png" sizes="180x180">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.
og-image-invalidog:image isn't a reachable, absolute, raster image within size limitsog-image-sizeog:image is smaller than 1200×630 or far from 1.91:1og-image-metaog:image:width/height missing or wrong, or og:image:alt missingtwitter-cardtwitter:card missingapple-touch-iconapple-touch-icon missing or not a 180×180 PNGmanifestWeb app manifest missing or incompletetheme-colorNo theme-color metarobots-txt-unreachablerobots.txt returns 5xx/429 or times out