When error bars are visible on a chart, the sidebar error bar content panel appears, but there isnt a 'more options' button in the content panel's title bar.
Markus Mohrhard committed a patch related to this issue.
It has been pushed to "master":
provide more button for chart error bar sidebar panel, tdf#95055
It will be available in 5.1.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.