fbgrab: 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/fbgrab - from package: fbcat

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

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