It means there is something that needs to be corrected on that Prebill prior to being able to generate an invoice. You need to highlight the Prebill record and then look at the detail panel on the right to check to see what the Defect Notes field has in it. This field will only display if there is an issue with a Prebill record.
For instance, it may say that the Matter has an unposted Invoice that must be posted prior to generating the next Invoice.
