If you are trying to duplicate an order and the order fails, it is probably because the gift (or print) that is being duplicated is out of stock at the moment.
Before ANYTHING else, go to Spree's homepage where the failed duplicate will be living. Click into the order (it will have no build information - it will be a totally blank order) and click into the customer. Change the customer to firstname.lastname@example.org - this is SUPER important. It removes the order from the customer's account and it helps us track orders without a print status.
Then contact the Customer Happiness Manager (or the Merch Manager if CHM is not available) to see if there are any extras in stock that can be used to fulfill the order. If the Manager says that there aren't, you then need to reach out to the customer and ask what gift they're like to replace it with (there's a macro for this) and offer some recommendations to help them out!
After they let you know, politely ask the CHM to go into Spree and add 1 to the stock of the Out of Stock item, solely for the purposes of duplication. Then, duplicate the build and change the gift to the new item that was selected. Make sure to Save!!
If the new item is not the same price as the original item that was paid for, you may need to issue a refund or request a payment. Reach out to the CHM for help with this (or look at the FAQ for how to issue a refund!)