qucsfilter: 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/qucsfilter - from package: qucs

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

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