apng2gif: 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/apng2gif - from package: apng2gif

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

Error in other languages:
apng2gif: bevel nie gevind nie
apng2gif: comandă negăsită
apng2gif: comando não encontrado
apng2gif: commande introuvable
apng2gif: command not found
apng2gif: käsku ei ole
apng2gif: không tìm thấy lệnh
apng2gif: komanda nerasta
apng2gif: Kommando nicht gefunden.
apng2gif: kommandot finns inte
apng2gif: komut yok
apng2gif: nie znaleziono polecenia
apng2gif: níor aimsíodh an t-ordú
apng2gif: no se encontró la orden
apng2gif: no s'ha trobat l'ordre
apng2gif: opdracht niet gevonden
apng2gif: parancs nem található
apng2gif: perintah tidak ditemukan
apng2gif: príkaz nenájdený
apng2gif: příkaz nenalezen
apng2gif: команда не найдена
apng2gif: командата не е открита
apng2gif: 命令找不到
apng2gif: コマンドが見つかりません