fls: 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/fls - from package: sleuthkit

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

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