fluxbox: 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 - from package: fluxbox

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

On Debian possible default full paths are:

/usr/bin/fluxbox - from package: fluxbox

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

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