GnuPG by default enforces secure access privileges, which means nobody else (but you) can access your GnuPG home directory ~/.gnupg. These access privileges often are not strict enough after copying the GnuPG home directory from another machine, and very often wrong ownership is the reason of such a message.
# Set ownership to your own user and primary group
chown -R "$USER:$(id -gn)" ~/.gnupg
# Set permissions to read, write, execute for only yourself, no others
chmod 700 ~/.gnupg
# Set permissions to read, write for only yourself, no others
chmod 600 ~/.gnupg/*
If you have (for any reason) created your own folders inside ~/.gnupg, you must also additionally apply execute permissions to that folder. Folders require execution privileges to be opened.