While trying to create a subscription via Zapier 2.0, you may face a challenge to not able to add Plan: Quantity(subscription_item[quantity]} sometimes. 

This is a mandatory field to enter while creating a subscription/adding an addon for flat_fee or per_unit price plans/items alone. Please check if the plan selected is a metered billing plan/addon if you are unable to enter it. In case of a metered billing plan or addon, this field is not mandatory.