fsls: 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/fsls - from package: python-fs

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

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