rdfs2dot: 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/rdfs2dot - from package: python-rdflib-tools

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

Error in other languages:
rdfs2dot: bevel nie gevind nie
rdfs2dot: comandă negăsită
rdfs2dot: comando não encontrado
rdfs2dot: commande introuvable
rdfs2dot: command not found
rdfs2dot: käsku ei ole
rdfs2dot: không tìm thấy lệnh
rdfs2dot: komanda nerasta
rdfs2dot: Kommando nicht gefunden.
rdfs2dot: kommandot finns inte
rdfs2dot: komut yok
rdfs2dot: nie znaleziono polecenia
rdfs2dot: níor aimsíodh an t-ordú
rdfs2dot: no se encontró la orden
rdfs2dot: no s'ha trobat l'ordre
rdfs2dot: opdracht niet gevonden
rdfs2dot: parancs nem található
rdfs2dot: perintah tidak ditemukan
rdfs2dot: príkaz nenájdený
rdfs2dot: příkaz nenalezen
rdfs2dot: команда не найдена
rdfs2dot: командата не е открита
rdfs2dot: 命令找不到
rdfs2dot: コマンドが見つかりません