Selecting different size is not changing the price

Store Design
1

Hi,
I’m using debut theme. The price is not changing automatically when I select different size in the product page.

https://wortheattt.myshopify.com/products/medium-veg-fruit-box

Why is that?

2

I found the reason, but I don’t know how to fix it, along with hiding the “add to cart button”

I hid the “add to cart” button by using Shopify instruction:

https://help.shopify.com/en/manual/online-store/themes/themes-by-shopify/vintage-themes/customizing-vintage-themes/hide-add-to-cart-buttons

This guideline in this link is not updated, after undo the given changes, everything came back to normal.

What can I do to hide my “add to cart” button and also not damage the size-pricing automatically issue?

Thanks