qucslib: 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/qucslib - from package: qucs

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

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