indexing.sitemap-missingThe site has no sitemap
Why it matters
A sitemap is the list of pages you want search engines to know about. It isn't required: Google says "If your site's pages are properly linked, Google can usually discover most of your site", and that you might not need one when the site is "small", which it puts at "about 500 pages or fewer", and well linked internally. It helps most when a site is large, is new with few links pointing at it, or has pages that aren't well linked together. It costs nothing, every CMS can write one, and it is how we know which pages to check: without it we follow links from the home page instead.
Source: Learn about sitemaps. So this is a notice on a small site and a warning once we find more than 500 pages. It is marked urgent because a sitemap that was there last week and has gone is usually a broken deploy.
How to fix it
Switch it on in your CMS or SEO plugin (WordPress has one built in at /wp-sitemap.xml; Yoast, Rank Math, Shopify, Squarespace and Wix all write one). Then tell crawlers where it is, in robots.txt:
Sitemap: https://www.example.com/sitemap.xmlWe look at the Sitemap: lines of robots.txt first, then /sitemap.xml.