I am using WordPress for a long time. For the first time, I have encountered such a problem. When I was trying to publish a post it was showing “publish failed”. Then I tried to save the draft of the post. Even then it was showing “save failed”. It was very annoying because I do not remember changing anything other than upgrading my WordPress version. Then I searched for a bit and found the solution.
I am using the http version of my site. But when I log in my dashboard it was showing https. So, I went to Settings>>General and changed my WordPress Address (URL) and Site Address (URL) to https. Then I tried to publish the post and it was successful. I do not know the reasons behind this problem but it solved my “publish failed” issue.