ethtool: 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:

/sbin/ethtool - from package: ethtool
/usr/sbin/ethtool - from package: ethtool

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

On Debian possible default full paths are:

/sbin/ethtool - from package: ethtool

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

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