Thanks for sharing on-point details about your concern, @dmckinny. 

Yes, you'll need to ask permission from your accountant to unclose the books for 2018. This way, you can delete the pending build assembly item by performing the steps provided by Rustler above. 

Before doing it, please first make sure to create a backup of your company file. Once deleted, you can close your 2018 books in the program. Since the deleted build assembly item is pending and it's a non-posting transaction, it doesn't affect the quantity on hand of components and assembly items for last year. 

For more information, you can check out this helpful article: Create, Build, and Work With Inventory Assembly Items. 

This will get you in the right direction, @dmckinny. 

I'll be right here to help if there's anything else you need. Have a great day.