cwebp: 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/cwebp - from package: libwebp-tools

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

On Debian possible default full paths are:

/usr/bin/cwebp - from package: webp

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

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