It looks like the Vimball plugin (which defines the :MkVimball
command) isn't active or installed. That's strange; it ships with Vim. Please investigate what's wrong with your setup (:set rtp?
and :scriptnames
will help), or re-install the plugin, e.g. from vim.org.
Or just install the clang_complete from a different format, e.g. the downloadable ZIPs from the vim-scripts.org mirror.