fnext: 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/bin/mh/fnext - from package: nmh

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

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