This article covers 

Why am I getting the "Cannot be Blank" error when using Chargebee's Cart-customization?
How do we pass a quantity parameter when the multi decimal is enabled for a site? 

Solution

Suppose Multi-decimal support is true, you may get the below error(screenshot) when you are trying to pass only a quantity parameter for per unit item_model.
 

This error occurs usually in cart_sub_create API.

As Multi_Decimal_Support is true you need to pass quantity_in_decimal parameter instead of quantity.

Related articles and Documentation

What is multi-decimal support in Chargebee? How does it work?

Configuring Multi-decimal Support