mheardd: 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/sbin/mheardd - from package: ax25-tools

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