heimdall-frontend: 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/heimdall-frontend - from package: heimdall-flash-frontend

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

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