qucsdigi: 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/qucsdigi - from package: qucs

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

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