img2sixel: 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/img2sixel - from package: libsixel-bin

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

Error in other languages:
img2sixel: bevel nie gevind nie
img2sixel: comandă negăsită
img2sixel: comando não encontrado
img2sixel: commande introuvable
img2sixel: command not found
img2sixel: käsku ei ole
img2sixel: không tìm thấy lệnh
img2sixel: komanda nerasta
img2sixel: Kommando nicht gefunden.
img2sixel: kommandot finns inte
img2sixel: komut yok
img2sixel: nie znaleziono polecenia
img2sixel: níor aimsíodh an t-ordú
img2sixel: no se encontró la orden
img2sixel: no s'ha trobat l'ordre
img2sixel: opdracht niet gevonden
img2sixel: parancs nem található
img2sixel: perintah tidak ditemukan
img2sixel: príkaz nenájdený
img2sixel: příkaz nenalezen
img2sixel: команда не найдена
img2sixel: командата не е открита
img2sixel: 命令找不到
img2sixel: コマンドが見つかりません