sixel2png: 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/sixel2png - from package: libsixel-bin

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

Error in other languages:
sixel2png: bevel nie gevind nie
sixel2png: comandă negăsită
sixel2png: comando não encontrado
sixel2png: commande introuvable
sixel2png: command not found
sixel2png: käsku ei ole
sixel2png: không tìm thấy lệnh
sixel2png: komanda nerasta
sixel2png: Kommando nicht gefunden.
sixel2png: kommandot finns inte
sixel2png: komut yok
sixel2png: nie znaleziono polecenia
sixel2png: níor aimsíodh an t-ordú
sixel2png: no se encontró la orden
sixel2png: no s'ha trobat l'ordre
sixel2png: opdracht niet gevonden
sixel2png: parancs nem található
sixel2png: perintah tidak ditemukan
sixel2png: príkaz nenájdený
sixel2png: příkaz nenalezen
sixel2png: команда не найдена
sixel2png: командата не е открита
sixel2png: 命令找不到
sixel2png: コマンドが見つかりません