rootn.exe: 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/rootn.exe - from package: root

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

On Debian possible default full paths are:

/usr/bin/rootn.exe - from package: root-system-bin

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

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