windowfilter: 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/windowfilter - from package: resample

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

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