Last some days, Joomla! released new version 3.4.2 and up to 3.4.3 right now, this version has some change with code, so the Helix II framework stopped working with Template Settings.
Right now, you can download a patch fix update for this problem via link below
[Helix II Template Framework for Joomla! 3.4.2 and 3.4.3]
After download, you install this patch update via Extension Manager. Last, you just clear your site cache (if you enable site cache), and go to Template Settings, so you can save it.
Note: If you still can’t save Template Settings, so you just go to Template Setting and click “Refresh” button in your browser, so you will possible to save it.