To support featured product sections, you have to add a code snippet.
This should be placed  inside the product form in sections/featured-product.liquid. 
If that template doesn’t exist, it may have another name.

Note: This is an instruction for OS 1.0 Themes.
For OS 2.0 Themes you have to add an App Block if possible.

Steps to follow:

Go to your Shopify Admin

1.    Navigate to Online Store

2.    Select Themes

3.    Navigate to Actions

4.    Click on Edit code

Search the sections/featured-product.liquid. 
Open the .liquid file and search the add to cart button.
Often the button is called like this: <button type=“submit“ name=“add“

Copy and paste the following code above the add to cart section:

<div class="textfieldapp" data-textfieldapp-product-json="{{ product | json | escape }}" data-textfieldapp-collections-json="{{ product.collections | json | escape }}"></div>