btdownloadgui.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/btdownloadgui.bittorrent - from package: bittorrent-gui

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

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