system-config-selinux: 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/system-config-selinux - from package: policycoreutils-gui

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