s9dupes: 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

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/s9dupes - from package: scheme9

You may install the required package with command apt-get install {package}
Error in other languages:
s9dupes: bevel nie gevind nie
s9dupes: comandă negăsită
s9dupes: comando não encontrado
s9dupes: commande introuvable
s9dupes: command not found
s9dupes: käsku ei ole
s9dupes: không tìm thấy lệnh
s9dupes: komanda nerasta
s9dupes: Kommando nicht gefunden.
s9dupes: kommandot finns inte
s9dupes: komut yok
s9dupes: nie znaleziono polecenia
s9dupes: níor aimsíodh an t-ordú
s9dupes: no se encontró la orden
s9dupes: no s'ha trobat l'ordre
s9dupes: opdracht niet gevonden
s9dupes: parancs nem található
s9dupes: perintah tidak ditemukan
s9dupes: príkaz nenájdený
s9dupes: příkaz nenalezen
s9dupes: команда не найдена
s9dupes: командата не е открита
s9dupes: 命令找不到
s9dupes: コマンドが見つかりません