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

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

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