I need an explanation.
in the config.toml file,
enableRobotsTXT must be placed inside, outside, before or after the [params] section?
Put it before.
just below this?
baseurl = "https://example.com/"
languageCode = "en-us"
languageLang = "en"
title = "title example"
theme = "themename"
Follow the advice at Requesting Help and share your repo and config. Then we can just look at it and test it. 
Robots. Plural. Your site is working correctly.
it is true, what a careless I am!
Ps. something I did not understand. in which folder the robots.txt and sitemap.xml files were created?
I believe robots.txt is at the root of the site directory, and the sitemap.xml too, but it might have other places it can live.
But to answer your question you should:
- Read up on
robots.txt - Read up on
sitemap.xml - Build a Hugo site and search your local filesystem to understand how they are rendered