http error 500

Hello,

Maybe a plugin or theme conflict. Please attempt to disable all plugins, and use one of the default (Twenty*) themes. If the problem goes away, enable them one by one to identify the source of your troubles.

As you don’t have access to Admin, you can do that via FTP
https://www.fixrunner.com/how-to-disable-wordpress-theme-using-ftp/

Thanks.