Are you trying to run revenue recognition for a period with zero hours? Projector used to expects there to be at least one time card, however small, in a recognition period. As of the Nov 2019 release, it now allows you to recognize revenue in a period with no time cards at all in recognition of the increasing prevalence of managed services contracts and retainer type work. It's more common with these sorts of contracts that a month may go by without your doing any work, but you still want to recognize some revenue.
When that happens, from an accounting perspective, any revenue or expense that would have routed to the resource cost center will route to the engagement's cost center.
From a reporting perspective reporting by the engagement cost center will be unaffected. Using the P&L cost center will show the behavior as described above. Using the resource cost center will show the revenue and expense, but attribute it to a <blank> cost center.