Hi again,
From digging into this, it looks like you cannot create demo payouts in Shopify. Payouts are generated automatically by Shopify Payments based on real transactions processed through the store. For testing purposes, you can use a development store to simulate other aspects of Shopify Payments, but payouts themselves cannot be manually created or simulated.
If your store has no payouts, the query above will return an empty result set. You can still test the mutation to ensure it executes without errors.