Hi,
SAP is failing to create a simulated billing document for budget billing extrapolation, so you need to check a few things if they are all set up correctly like, whether the rate category and rate type is consistent for the installation, possibly try creating a simulation billing document and check if it has any posting relevant normal billing line items(for which extrapolation line items are created during PS creation). Also check that BBP 4 is set at contact account level (I think there is no problem with this, would have got a different error otherwise.).
Thanks ,
Pawan