Solve “Unable to find a sitemap to generate” in Magento
If Magento stopped generating sitemap suddenly when trying to generate a Google Sitemap for an existing entry in Magento make sure to check that the sitemap ID generated is set correctly, i.e. not equal to 0 (zero).
In this particular Magento 18.104.22.168 setup the ID for the first sitemap wrongly was set to 0 on generation via Catalog / Google Sitemap, thus breaking the sitemap generation process through the Generate option.
Manually setting the sitemap ID in the database to 1 for the first sitemap solved this problem.