gpg-mailkeys: 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/gpg-mailkeys - from package: signing-party

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

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