seqdiag3: 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/seqdiag3 - from package: python3-seqdiag

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

Error in other languages:
seqdiag3: bevel nie gevind nie
seqdiag3: comandă negăsită
seqdiag3: comando não encontrado
seqdiag3: commande introuvable
seqdiag3: command not found
seqdiag3: käsku ei ole
seqdiag3: không tìm thấy lệnh
seqdiag3: komanda nerasta
seqdiag3: Kommando nicht gefunden.
seqdiag3: kommandot finns inte
seqdiag3: komut yok
seqdiag3: nie znaleziono polecenia
seqdiag3: níor aimsíodh an t-ordú
seqdiag3: no se encontró la orden
seqdiag3: no s'ha trobat l'ordre
seqdiag3: opdracht niet gevonden
seqdiag3: parancs nem található
seqdiag3: perintah tidak ditemukan
seqdiag3: príkaz nenájdený
seqdiag3: příkaz nenalezen
seqdiag3: команда не найдена
seqdiag3: командата не е открита
seqdiag3: 命令找不到
seqdiag3: コマンドが見つかりません