heartbleeder: command not found

Have You tried accessing the binary with the full path?

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/heartbleeder - from package: heartbleeder

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