ffplugingui: command not found

Have You tried accessing the binary with the full path?

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/ffplugingui - from package: fawkes-guis

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