Susanna: How do I enable entry of this item?
My post above covers QuickBooks Online but this reply will cover QuickBooks Desktop.
It sounds like this item was used in an invoice, in this case your item was never deleted in QuickBooks, it was simple made inactive. If you look at the QuickBooks tab under the Resolve Conflicts tab link you will probably have a conflict to resolve. What you want to do is refetch the item from QuickBooks and it will restore it in Method.
~C