Payment terms is used to make payment schedules, which is a feature of the B2B product. Do you use that? Or do you use some other system?
Reading through the above thread, it seems like your are trying to reproduce the B2B payment terms feature through Flow. In order for “Send Payment Reminder” to work, it needs a Payment Schedule. So you need to first create that payment schedule for an order.
I believe the way to create a schedule is to call this the create API for https://shopify.dev/docs/api/admin-graphql/2023-07/objects/PaymentTerms. You would need to use Send HTTP Request to call this particular API.