system-config-network: 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/system-config-network - from package: system-config-network-tui
/usr/sbin/system-config-network - from package: system-config-network-tui

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

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