I found this old message from 2013:
I tried the suggestion but it didn’t help.
Setup -> Customize -> Customize Form -> BOM Item
“Mandatory” was checked, I deselected it, saved, and then tried changing my BOM to remove quantity. I still get the “Item Quantity needed for…” popup when I try to save.
Note: The BOM Item popup would always let me enter and click “Done” for an item with zero-quantity, but I could never save the resulting BOM after it was built. That’s where the popup with “Item Quantity needed for…” would come up. It seems somewhere else in the system it’s checking those quantities.