Thx @maiki for your help.
Do you have a page with headings that isn’t a chapter? If so, it should show a TOC.
As far as I understood the documentation provided by your link every page is a non-chapter page (called “default” page) as long as I don’t provide chapter = true.
This is the case for all my pages (no chapter = true given) but I haven’t a TOC in any of my pages. 