Thank you for writing in, maybe that was hidden intentionally.
Take a look on this Remove Theme Editor, some hosting do hide that editor by default, so you might want to ask your hosting support about that as well.
But we don't recommend editing theme or plugin file in there though, because you can easily lockup on your site with the slightest mistake you made on a php file.
Access your files via FTP instead, that way if you made a mistake you can correct it immediately.
First things first, I don't see you install or activate a child theme on your site, please set up a child theme first.
Setup - How To Setup Child Themes
Let us know how it goes,