blockdiag: 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/blockdiag - from package: python-blockdiag

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

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