This is just a friendly tip to others who might, like me, be wandering around not finding answers that are actually helpful and solve the problem at hand.
In my specific case, I got this error when trying to delete my index. On the relevant docs page, https://www.elastic.co/guide/en/elasticsearch/reference/6.0/indices-delete-index.html, the code example simply says:
DELETE /twitter
But I had always used curl, as in the other examples I'd seen, so that's what I did here:
curl -XGET 'localhost:9200/DELETE/ktab'
And that's what started the errors flowing. I googled the error message, but did not find any useful help. Then the response to one post on this forum struck me. The responder told the OP that you can't use _bulk in the browser. That struck me because I wasn't using the browser (I wasn't using _bulk either, but bear with me. I like tangents. My sister calls them 'rabbit holes' 
. ) I went back to the delete page in the docs and stared at the code example. Why did it only say 'DELETE /twitter'? Where was the curl?
Thankfully, there is a link to get the curl version in the bottom of a lot of these code examples, so I clicked it and pasted it into a text editor to have a look:
curl -XDELETE 'localhost:9200/twitter?pretty'
Not sure why a simple delete needs to be pretty, but that's beside the point. As I hope you have noticed, this code is quite different from the example. I have only rarely had the need to use curl before now, but obviously the command was wrong. The more important realization to me, however, is that this documentation was assuming that I WAS using the browser. (kibana stopped working for me the second day I had it, but that too is a digression/rabbit hole).
Knowing that makes all the difference in the world in what examples and commands and syntax work, and which ones don't. So keep that in mind, fellow newbies, and good luck to you all.