I was getting this error after installing the HTML Publisher:

[htmlpublisher] Archiving HTML reports...
[htmlpublisher] Archiving at PROJECT level
/var/lib/jenkins/workspace/simplesimple/reports/html/ to /var/lib/jenkins/jobs/simplesimple/htmlreports/HTML_Report
ERROR: Specified HTML directory '/var/lib/jenkins/workspace/simplesimple/reports/html/ ' does not exist.
Build step 'Publish HTML reports' changed build result to FAILURE

The problem was, I had an additional space character at the end of my directory entry, which wasn't obvious from the interface.

It would be great to update this to trim leading/trailing whitespace from directory names.