tmux: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/tmux - from package: tmux

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/tmux - from package: tmux

You may install the required package with command apt-get install {package}

Error in other languages:
tmux: bevel nie gevind nie
tmux: comandă negăsită
tmux: comando não encontrado
tmux: commande introuvable
tmux: command not found
tmux: käsku ei ole
tmux: không tìm thấy lệnh
tmux: komanda nerasta
tmux: Kommando nicht gefunden.
tmux: kommandot finns inte
tmux: komut yok
tmux: nie znaleziono polecenia
tmux: níor aimsíodh an t-ordú
tmux: no se encontró la orden
tmux: no s'ha trobat l'ordre
tmux: opdracht niet gevonden
tmux: parancs nem található
tmux: perintah tidak ditemukan
tmux: príkaz nenájdený
tmux: příkaz nenalezen
tmux: команда не найдена
tmux: командата не е открита
tmux: 命令找不到
tmux: コマンドが見つかりません