btdownloadheadless.bittorrent: 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/btdownloadheadless.bittorrent - from package: bittorrent

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

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