fv: 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/fv - from package: ftools-fv

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