fluxbox-generate_menu: 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/fluxbox-generate_menu - from package: fluxbox

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