apngdis: 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/apngdis - from package: apngdis

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

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