Fenv: 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/Fenv - from package: ferret-vis

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