What to run
Open Robots.txt Generator. It stays in the browser.
How to read the result
The file is text for https://example.com/robots.txt.
Disallow: / blocks crawlers that honour it.
It is not access control.
A sitemap line does not create the sitemap.
What to open next
Open HTTP Headers or Is This Website Down on /robots.txt after you upload. Both checks run from Cloudflare.
Open htaccess Redirect Generator if the file must live at the root via a redirect. Usually it should not. The generator stays in the browser.