The server commands are defined in User_Group:
nChatMessage in the source.
"[voting system] Voting not enabled" is displayed if "!vote" is issued and m_voting_threshold > 100 || m_voting_threshold < 1.
m_voting_threshold defaults to 110, I think...
And that's likely changed by uncommenting this line in the config file:
# SetVotingThreshold 50 # sets threshold to 50%. can be 1-100%, or >100 to disable
(It would have been nice if the comment said "disabled by default" to make it clear.)