Hi,
Select distribution formula as 'Object' and select the appropriate WBS at the RECN transaction. The accrual / deferral posting should work fine in this case.
Please make sure, that distribution is allowed for WBS for the contract type that you are using at SPRO->Flexible Real Estate Management (RE-FX)->Contract->Objects->Permitted Object Types per Contract Type.
Alternatively, for existing contracts, you can also set the appropriate WBS as receiver cost object at RECN->Contract Change Mode->Edit->CO Settlement Rule. For this you will have to run the settlement engine separately after you run the accrual engine. Using the transaction RECOSESNG or RECOSEVAR.
Hope this helps..
Regards,
Hardik Sharma