etrn: command not found

Have You tried accessing the binary with the full path?

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/sbin/etrn - from package: sendmail-base

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