To solve the issue, when you press the button it should convert to a token card. Afterwards, you have to create a separate workflow “When a token card was created” where you create customer and then save Customer ID and Card ID to the database.
Then use these IDs in your charge calls. Before using “Seller charge a customer” call, you have to use “Link customer with seller” first, and from the obtained result you should get the ID of payment source.
To make things simple, please check again the plugin documentation and follow all steps to make sure configurations have been done properly, especially pay attention to the moment when adding a card. Also, check demo app here https://zeroqode-demo-19.bubbleapps.io/stripe_marketplace and the editor here https://bubble.io/page?type=page&name=stripe_marketplace&id=zeroqode-demo-19&tab=tabs-1 to verify the settings.
Hope it is helpful!