dwebp: 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/dwebp - from package: libwebp-tools

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

On Debian possible default full paths are:

/usr/bin/dwebp - from package: webp

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

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