xcf2png: 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/xcf2png - from package: xcftools

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

Error in other languages:
xcf2png: bevel nie gevind nie
xcf2png: comandă negăsită
xcf2png: comando não encontrado
xcf2png: commande introuvable
xcf2png: command not found
xcf2png: käsku ei ole
xcf2png: không tìm thấy lệnh
xcf2png: komanda nerasta
xcf2png: Kommando nicht gefunden.
xcf2png: kommandot finns inte
xcf2png: komut yok
xcf2png: nie znaleziono polecenia
xcf2png: níor aimsíodh an t-ordú
xcf2png: no se encontró la orden
xcf2png: no s'ha trobat l'ordre
xcf2png: opdracht niet gevonden
xcf2png: parancs nem található
xcf2png: perintah tidak ditemukan
xcf2png: príkaz nenájdený
xcf2png: příkaz nenalezen
xcf2png: команда не найдена
xcf2png: командата не е открита
xcf2png: 命令找不到
xcf2png: コマンドが見つかりません