heimdall: 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 - from package: heimdall-flash

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

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