Are you exporting the site and using your own hosting solution or are you hosting on Webflow? Webflow will create a robots.txt file automatically if you have content typed in the SEO > Robots.txt section for hosted sites.
Under Site Settings, press SEO:
Scroll down to Robots.txt and enter your content. You can type Allow: / if you want a vanilla Robots.txt entry.
Upon publishing, you’ll have a Robots.txt file published right inside the root directory of your site!
Edit: This was added to the Wishlist section a while back: Create more options, standard text, for Robots.txt - Product feedback - Forum | Webflow