Comment 1 for bug 841867

This is a guess as I'm not familiar with the code there, but it sounds like Kupfer couldn't find any candidates for a terminal emulator on your system. What do you use for a terminal?

This will probably work as a workaround:
in file /usr/share/kupfer/kupfer/core/settings.py, add the following line at line 396 (the line right after the """s for get_valid_alternative_ids):

if not category_key in self._alternative_validators: return

The equivalent patch is attached, although this doesn't address the issue that Kupfer wasn't able to find a terminal. You'll have to configure it yourself using the Custom Terminal plugin.