fiwalk: 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/fiwalk - from package: sleuthkit

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

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