Why isn't the Line Item Property functioning in Dawn theme?

I’m currently developing a site where “Line Item Property” is not working, and I was wondering if you could tell me why.

【question】 Line Item Property does not work (Dawn)

(1) Generate code with UI Element Generator

(2) Insert the code in main-product.liquid

  1. Add form=“product-form-{{ http://section.id }}”.

Theme: Dawn

It didn’t work here.
I tried to add it in the “form tag” but it didn’t work.

I tried it on another site, Dawn, but it didn’t work.