rdflib: 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 CentOS possible default full paths are:

/usr/bin/rdflib - from package: nasm-rdoff

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/rdflib - from package: nasm

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

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