mtail: 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/bin/mtail - from package: monkeytail

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