User had set up a recurring bill in Core that would automate the process of creating vendor bills at a fixed frequency, but the process stopped after some time and Core does not generate the recurring bills any more.
This can happen if the vendor bill is associated with projects whose status was changed to something other than Active. Core no longer processes recurring bills if the source vendor bill has projects whose status is not Active. Make sure all the associated projects have active status if you want to continue generating the recurring bills. Check the Core Help for details.