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

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

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