Why is the XUnit plugin not working with Jenkins?
I was getting this error too when I first enabled the XUnit plugin.
I hadn't realized (or had forgotten) that I needed to add a report type to the XUnit build step configuration. To do so, click Add and then select the type of report you want parsed (for me it was PHPUnit-3.x). See this screenshot. You'll also have to specify the path to the XML report files that you want parsed. (I'm using Codeception, so it was tests/_log/*.xml.)