conmux-attach: 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/conmux-attach - from package: conmux

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

On Debian possible default full paths are:

/usr/bin/conmux-attach - from package: conmux

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

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