mewdecode: 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/mewdecode - from package: mew-beta-bin
/usr/bin/mewdecode - from package: mew-bin

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

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