byobu-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/byobu-tmux - from package: byobu

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

On Debian possible default full paths are:

/usr/bin/byobu-tmux - from package: byobu

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

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