Couldn't generate PGP key, general error
Solution 1:
The reason that error was occurred are following:
~/.gnupg/*.gpg files had read-only permissions.
Before I use this operation, I created a gpg key by using following commands.
sudo -s
gpg --gen-key
After that, I logged in regular user.
Thus, I hadn't the permissions to write ~/.gnupg/*.gpg (it's owner is root) files.
The solution: I modified it's permission by using following commands so that I created gpg key successfully.
sudo -s
chown xxx ~/.gnupg/*.gpg