metapixel-sizesort: 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/metapixel-sizesort - from package: metapixel

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

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