This really nothing has nothing to do with DokuWiki, but I'll answer it in the hope that others with the same question will look for the answer before posting a question best answered by Google.
sam01 wrote
Where is robots.txt in dokuwiki?
Nowhere.
robots.txt is specific to a particular web-server. The developers of DokuWiki have no way of knowing what you want search engines to index - or even what you are serving with your web-server.
tl;dr? There isn't one that comes with DokuWiki because it's impossible to write a correct one without knowing what you have and what you want indexed (by search engines).
sam01 wrote
how make?
Create it with a text editor. It must be called robots.txt, be readable by your web-server (and not others), and not be writable. It must reside in the web-server document root.
sam01 wrote
what i write in robots.txt?! and all about robots.txt...
https://en.wikipedia.org/wiki/Robots_exclusion_standard (available in other languages)
https://support.google.com/webmasters/answer/6062608?hl=en (also available in other languages)
http://www.robotstxt.org/robotstxt.html