Forgive the lengthy post. It is fixed for me now and I’m no longer getting the Missing XML Tag on Google Webmaster Tools (GWT). I’m not exactly sure “what” fixed it, but here are some steps I took… maybe someone else can glean something from it ??
In my case, on GWT, the error was referring to the “Category” and “Post_Tag” xml’s. Also, it indicated on both that there was an error on LINE #3 of the xml’s.
I didn’t have any Categories or Posts (all my pages are static) so I added one of each. In SEO/SiteMaps, I clicked the XML Sitemap button and could view my Sitemap, including sub-maps for the Category and Post_Tag and could open & view each of them independently in the browser.
Back to GWT, tried resubmitting the sitemap but it resulted in the same error.
I then went into SEO/ExcludeTaxonomies and placed checkmarks next to Categories & Tags. Save Changes. Clicked XML Sitemap and this time it did not generate the Category and Post_Tag sub-maps.
Back to GWT, resubmitted, but STILL had the same problem. I don’t know “what” GWT was seeing. Were there leftover xml’s for those two sub-maps? (note: I’m not using a wordpress cache plugin)
I installed the Plugin “Google XML Sitemaps”, generated its Sitemap, and was able to submit it successfully at GWT. Tried YoastSEO’s Sitemap again and still no joy.
Back to SEO/ExcludeTaxonomies, removed checkmarks, saved, resubmitted, same problem at GWT.
Finally, back to SEO/ExcludeTaxonomies, placed checkmarks back on Categories and Post_Tag, saved, resubmitted to GWT and all was well this time.
So, again, I’m really not sure “what” fixed it. I wonder if it was toggling the ExcludeTaxonomies check boxes (saving in between) that did it.
GWT definitely did not like line #3 in both the Categories.xml and Post_Tag.xml