webpmux: 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 CentOS possible default full paths are:

/usr/bin/webpmux - from package: libwebp-tools

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/webpmux - from package: webp

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

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