Why is the Add product button grayed out?
If you are trying to add a subscription product to a customer's subscription, the Add product button is grayed out. This is due to having dynamic pricing set up.
You cannot add a subscription product to an existing customer's subscription through the merchant portal if you've only assigned a dynamic pricing plan to that product. You can only add these types of products as one-time purchases to existing subscriptions.
In order to avoid this, you must add additional plans that are not utilizing dynamic pricing to a product that is currently assigned a dynamic pricing plan. Be sure to set the product plan settings to not available on the subscription widget if you do not want the regular subscription plan to display on the storefront.