This is a known error in QuickBooks, so a fix should be out shortly, so this may or may not apply to you.

The Billable Expenses error happens when trying to add Additional Billable Expenses to an Existing Invoice. There is no problem adding to a new invoice.

Here are two work-arounds until Intuit fixes the issue.

1. If the invoice has no existing billable expenses:  Make a copy (from the More menu at the bottom) of the original invoice, then add the billable expenses, and delete the original invoice.
2. If the invoice has existing billable expenses: Delete one of the invoice line items that represents a billable expense, confirm the deletion and unlinking of that expense from the invoice, now all the unbilled expenses will appear in the Add to Invoice side bar for you to select and add to the invoice.

Thanks to Intuitive Accountant contributing author Liz Scott for those tips.
http://www.intuitiveaccountant.com/general-ledger/quickbooks-online-known-error-existing-invoices-with-billabl/#.V9gvWvkrKUk