fbrun: 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/fbrun - from package: fluxbox

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

On Debian possible default full paths are:

/usr/bin/fbrun - from package: fluxbox

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

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