Thanks for reaching out, I’d be happy to help you.
For BOM products, the backorders field is only available when they are set as "Sellable"
So, if you want all your products to be sellable you can enable it in ATUM Inventory > Settings > Product Levels > Add selling of BOM.
If you want to sell only certain products, please, go to the product individual option and configure it as sellable: Product Data > ATUM Inventory > Product Levels Settings > Make Sellable.
This is done this way because the backorders option only makes sense for sales from the front. If a BOM is not sellable, you don't need this option at all.
If there’s anything else I can help you with, please let me know.