We are experiencing an issue with regards to our existing Developer Preview plan. When I create a draft order and mark it as paid. I am getting this message saying:
To save this draft order, 1 change needs to be made:
This used to work for us but that it isn't the case anymore.
Sophia here from Shopify.
There was an update regarding test orders for development stores that was posted to the developer changelog on March 30, 2020. I recommend subscribing to RSS on that page so you don't miss any future updates.
The current process for testing orders on development stores is to either use Shopify's Bogus Gateway or enable test mode for your payment provider. In any case, you can process unlimited test transactions. The full details including step-by-step instructions can be found here: Testing orders in development stores.