We had a namespace in fawikibooks equal to "Subject" with the index number 112 equal to enwikibooks, new namespace with the index number 2600 in Flow extension which were added in Ib8cc18d9e478ab98799a3bc79358387ebc38b338 had the same name, due to conflict it wasn't listed, I have fixed the name in I88d059dae86eba42b1bf8c69b09a0f992373c0bd now is up, but it's not listed yet.
|operations/mediawiki-config : master||Re-enable Flow for fawikibooks|
Checked Special pages - Flow is listed there. https://fa.wikibooks.org/wiki/%D9%88%DB%8C%DA%98%D9%87:%D8%B5%D9%81%D8%AD%D9%87%E2%80%8C%D9%87%D8%A7%DB%8C_%D9%88%DB%8C%DA%98%D9%87
Note: http://quarry.wmflabs.org/: use fawikibooks_p gives 0 results for page.page_content_model='flow-board' and revision.rev_content_model='flow-board'.