ninjahelper: 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/sbin/ninjahelper - from package: backupninja

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

On Debian possible default full paths are:

/usr/sbin/ninjahelper - from package: backupninja

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

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