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

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

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