User has accidentally merged a sub-phase with the parent project. The hours of the sub-phase mixed with hours of the parent project cannot all be moved as some of the hours have already been billed.
The best way to handle this is to start by downloading the invoices that have already been created to distinguish between the hours billed and those that have have not been billed. Delete these invoices, which frees the associated time entries. Move the entries to the appropriate parent project or sub-phases, as needed. Recreate the invoices making sure to use the same invoice numbers.