fb: 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 Debian possible default full paths are:

/usr/bin/fb - from package: findbugs

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

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