byobu-ctrl-a: 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-ctrl-a - from package: byobu

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

On Debian possible default full paths are:

/usr/bin/byobu-ctrl-a - from package: byobu

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

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