ryu-manager: 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 Debian possible default full paths are:

/usr/bin/ryu-manager - from package: ryu-bin

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

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