[Solved] Why the "serve" slot won't work for some objects?

When it's a food stall, the "_ctnm_serve_0_0" works normally, as expected.



I'm trying to add a serve position to the flower stall, and it won't work:



It appears as this pink triangle.
I added an option for the vendor to sell food (in the flower stall tuning), but they won't serve since they can't "recognize" there's a slot to serve the food. 
The vendor, in one of my tests, closed the flower stall and served me on another food stall nearby. So they can serve. The only problem that the "_ctnm_serve_0_0" on the flower stall is nor being recognized.

I tried adding the "_ctnm_serve_0_0" slot to other objects (that are not food stalls/cafeterias) and it also didn't work.
They only work on objects that actually serve food.


How can I fix that? What makes the "_ctnm_serve_0_0" work? Does anybody know? I couldn't find this information anywhere.

Thanks for any help.