quassel: 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/quassel - from package: quassel-kde4
/usr/bin/quassel - from package: quassel

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

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