I am not sure I would have enough information to answer this question, but I do not want to leave you hanging, and I appreciate you reaching out. So..
A 500 Internal Server error could be for a variety of reasons. Due to the fact that you are stating that you are using WordPress, the most common reason for this is usually a conflict with theme/plugin/php version. I would try to disable plugins, change theme to default, and make sure .htaccess, .ini files are not creating this. Check what version of php you are using, maybe create a phpinfo.php file, or under cPanel -> go to Software -> and click 'Select PHP Versoin', where you can check what version is run.
Also since you mentioned public_html, I am assuming you are using cPanel? You created an 'Addon Domain' inside cPanel, and then installed WordPress to it manually? Does this also mean that WordPress site you are talking about has already been built and now is being migrated? I have many questions to best help you, please provide more specifics.
Hope this helps!
William Sullivan - GoDaddy | Community Moderator 24/7 support available at x.co/247support P.S. - If you find my answer helpful, please feel free to give kudos.