mkThumbs: 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/mkThumbs - from package: oggvideotools

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

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