theora_png2theora: 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/theora_png2theora - from package: libtheora-bin

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

Error in other languages:
theora_png2theora: bevel nie gevind nie
theora_png2theora: comandă negăsită
theora_png2theora: comando não encontrado
theora_png2theora: commande introuvable
theora_png2theora: command not found
theora_png2theora: käsku ei ole
theora_png2theora: không tìm thấy lệnh
theora_png2theora: komanda nerasta
theora_png2theora: Kommando nicht gefunden.
theora_png2theora: kommandot finns inte
theora_png2theora: komut yok
theora_png2theora: nie znaleziono polecenia
theora_png2theora: níor aimsíodh an t-ordú
theora_png2theora: no se encontró la orden
theora_png2theora: no s'ha trobat l'ordre
theora_png2theora: opdracht niet gevonden
theora_png2theora: parancs nem található
theora_png2theora: perintah tidak ditemukan
theora_png2theora: príkaz nenájdený
theora_png2theora: příkaz nenalezen
theora_png2theora: команда не найдена
theora_png2theora: командата не е открита
theora_png2theora: 命令找不到
theora_png2theora: コマンドが見つかりません