mewencode: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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

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