guessnet-ifupdown: 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 Debian possible default full paths are:

/usr/sbin/guessnet-ifupdown - from package: guessnet

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

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