twpsk: 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/twpsk - from package: twpsk

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

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