Can’t find theme file editor under appearance

I am trying to add a code that will help to make my product photos clearer. I am told that I need to go to theme file editor, under appearance. This is where I should be able to add the code but it doesn’t seem to exist. In another video to fix this issue, I am told to go to file manager and add a word of code to the wp-config.php file. But everytime I try to add the word “false” in replacement of the word “true”, I get a message that looks like the one I put at the bottom. I am starting to feel hopeless as I have now been watching other videos on how to fix this error. I am getting no where on my own it appears. Please help. Thank you!

Unable to connect to backend.
Access denied.
HTTP error 403