Submitted my sitemap and Search Console immediately showed “Sitemap could not be read” — 0 pages discovered. Before rebuilding anything, verify what Google actually gets:
curl -s -o /dev/null -w "%{http_code} %{content_type}\n" \
-A "Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)" \
https://example.com/sitemap-index.xml
Mine returned 200 application/xml with valid XML — nothing was wrong. The error is just GSC displaying its first queued/failed fetch attempt, which is common right after submission (and especially if you submitted mid-deploy). It resolves on its own within a day or two.
Also learned: the sitemap report’s “Last read” date lags even after success. The Pages report under Indexing is the truthful view.