byobu-ulevel: 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-ulevel - from package: byobu

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

On Debian possible default full paths are:

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

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

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