PayPal Checkout: got an error message to say "ORDER ID is missing"

That all looked fine, but when I tried to enter a couple of people for the event, I got an error message to say "ORDER ID is missing from the form submission".  But I can't see anywhere to add in an order ID.  

Usually, this error means that payment was not processed. Our current PayPal Checkout integration works this way: 

- a user must press a PayPal button and complete a payment;

- then, user can submit a form. 

This may be changed later to exclude an extra step, but for now, I can suggest to place a warning for your users to press PayPal button: 

Thank you.  I think that's cracked it. :)