Invalid Reference Id : Categories cannot be used in transactions.
May also say:
Invoice is using an item that has sub-items. Such items are called categories in QBO and are not allowed on invoices.
Items that are used on invoices cannot have sub-items
This error occurs when a bill or invoice with items on it in Bill.com is trying to sync to QuickBooks Online (QBO), but one or more of the items on the bill/invoice is a "category" type item in QBO.
Edit the bill/invoice in Bill.com to have items that are not of the category type, save and sync.