quasselcore: 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/quasselcore - from package: quassel-core

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

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