mailman-update-cfg: 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/mailman-update-cfg - from package: mailman

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