I don't understand the language but the UI on that page is of Post Editor, not the Site editor.
Site editor, or in other manner theme editors are provided under the Appearance settings. You should look under that option, also it requires that you have purchases the theme before you can edit it. Otherwise you cannot edit the theme, or you have to write a theme from scratch.
For more, please read Wordpress documentation.
https://codex.wordpress.org/Editing_Files[
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