User is trying to set the Billable flag for some non-billable time entries, but cannot do so and gets a message that the entries are already billed.
This happens if the non-billable entries are linked to an invoice. When you create a batch invoice, the non-billable entries also get linked to it by default and are marked as billed. You cannot update the Billable flag for such entries. If you want to update this flag, you need to delete the invoice to which the entries are linked and then mark them as billable. To know the invoice number of such an invoice, you must show that column in the Time Entries screen by clicking More > Show/Hide Columns > Invoice Number. You can always recreate the invoice after you have changed the Billable flag for the entries. Check the Core Help for details on deleting invoices.