png23d: command not found

Have You tried accessing the binary with the full path?

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/png23d - from package: png23d

You may install the required package with command apt-get install {package}
Error in other languages:
png23d: bevel nie gevind nie
png23d: comandă negăsită
png23d: comando não encontrado
png23d: commande introuvable
png23d: command not found
png23d: käsku ei ole
png23d: không tìm thấy lệnh
png23d: komanda nerasta
png23d: Kommando nicht gefunden.
png23d: kommandot finns inte
png23d: komut yok
png23d: nie znaleziono polecenia
png23d: níor aimsíodh an t-ordú
png23d: no se encontró la orden
png23d: no s'ha trobat l'ordre
png23d: opdracht niet gevonden
png23d: parancs nem található
png23d: perintah tidak ditemukan
png23d: príkaz nenájdený
png23d: příkaz nenalezen
png23d: команда не найдена
png23d: командата не е открита
png23d: 命令找不到
png23d: コマンドが見つかりません