The version fetched with apt-get is not up to date. Some of the tips in the tmux 2 book depend on the newer 2.3 version.
sudo apt-get install -y software-properties-common
sudo add-apt-repository -y ppa:pi-rho/dev
sudo apt-get update
sudo apt-get install -y software-properties-common tmux-next
echo "alias tmux=tmux-next" >> ~/.bashrc