User is trying to bill some projects in the Create Batch Invoice screen by joining them, but gets the following prompt:
One or more joint rows selected without selecting parent row. Please select parent row to continue.
This happens if you try to bill projects (or their phases) from combined rows without selecting the parent row in the Create Batch Invoice screen. You can combine the rows manually or using the project rule Send as combined invoice, and the combined rows appear under a parent row that has the client name (or root project name, if you have used the Global Settings > Billing rule Restrict combined invoices to root projects). After you combine the rows, you cannot bill individual items from them. You can either bill the combination or you need to first separate out the projects to bill by selecting them and clicking Actions > Separate. Check the Core Help for details.