Vouchers appear as a payment method when the cart includes products that have a
mollieProductCategory value assigned.
mollieProductCategory can be assigned both as a product and as a payment method attribute. If you have assigned a value to both, the product attribute takes precedence the payment method attribute.
mollieProductCategory attribute for products in Salesforce Commerce Cloud under Merchant Tools > Products and Catalogs > Products > (select product) > Mollie product category.