qucs: 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 CentOS possible default full paths are:

/usr/bin/qucs - from package: qucs

You may install the required package with command yum install {package}

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