post_tag-sitemap.xml don’t work

A few things to isolate before treating this as a general sitemap failure:

1. Open the child sitemap directly:
   post_tag-sitemap.xml
2. Check whether the tag taxonomy actually has indexable terms:

• tags exist
• tags are attached to published posts
• the tag archive is not set to noindex
• the individual tag terms are not set to noindex

If there are no indexable tag terms, the post_tag sitemap can fail or appear missing even when “Include in Sitemap” is enabled.

3. Flush the sitemap/rewrite state:

• Rank Math > Sitemap Settings: turn sitemap off, save, turn it on, save
• Settings > Permalinks: save once without changes
• clear page cache/CDN/object cache

3. Exclude sitemap URLs from caching if you use a cache/CDN layer:
   sitemap_index.xml
   *-sitemap.xml

If only the post_tag sitemap fails while other child sitemaps work, I would focus on taxonomy/indexability/caching first rather than the main sitemap index.