aprsmon: 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/aprsmon - from package: aprsdigi

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