Try exiting it and then make sure no tmux process is still running, by for example running ps -aux | grep tmux.
For future reference: the command to kill the tmux daemon (and as a side-effect, all other running tmux processes connected to it) is tmux kill-server
(or in tmux, typing <prefix> :kill-server
, assuming default keybindings).
In the Netherlands it doesn’t even have to be a valid ID. If it hasn’t been expired for more than 5 years it’s fine for voting purposes.