fluxbox-update_configs: 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/fluxbox-update_configs - from package: fluxbox

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

On Debian possible default full paths are:

/usr/bin/fluxbox-update_configs - from package: fluxbox

You may install the required package with command apt-get install {package}

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