XSupplicantUI: 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/XSupplicantUI - from package: xsupplicant-ui

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

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