User has marked unbilled time entries of a project as billed using Batch Update and now wants to batch update the Invoice Number for those entries, but cannot find any invoice number in the drop-down.
You can batch update the Invoice Number for entries only to an existing one, so you must have at least one invoice on the project for it to be visible in the drop-down. You cannot assign any random number when using the Batch Update option; you need to select one from the existing invoices.
Note: You should use the Batch Update option on time entries for the invoice number only when you want to do Progress Billing, as the entries do not add anything to the invoice amount. Core only links them to the invoice and automatically writes them up or down to adjust the difference from the invoice amount. Check the Core Help for details. You may also want to check out this article: How to attach time entries to an existing invoice.