Can I edit the "robots.txt" file?

Unfortunately, you cannot modify the "robots.txt" file at the moment.

The only way to make changes is to export the project to your custom hosting and make the necessary changes over there.

If you want to add or remove "disallow", go to Page settings → Facebook & SEO → Appearance in search results → Customise appearance on search → check (or uncheck) the box "Forbid search engines from indexing this page".