apngopt: 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/apngopt - from package: apngopt

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

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