blockdiag3: 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/blockdiag3 - from package: python3-blockdiag

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

Error in other languages:
blockdiag3: bevel nie gevind nie
blockdiag3: comandă negăsită
blockdiag3: comando não encontrado
blockdiag3: commande introuvable
blockdiag3: command not found
blockdiag3: käsku ei ole
blockdiag3: không tìm thấy lệnh
blockdiag3: komanda nerasta
blockdiag3: Kommando nicht gefunden.
blockdiag3: kommandot finns inte
blockdiag3: komut yok
blockdiag3: nie znaleziono polecenia
blockdiag3: níor aimsíodh an t-ordú
blockdiag3: no se encontró la orden
blockdiag3: no s'ha trobat l'ordre
blockdiag3: opdracht niet gevonden
blockdiag3: parancs nem található
blockdiag3: perintah tidak ditemukan
blockdiag3: príkaz nenájdený
blockdiag3: příkaz nenalezen
blockdiag3: команда не найдена
blockdiag3: командата не е открита
blockdiag3: 命令找不到
blockdiag3: コマンドが見つかりません