Hi Fita,
Based on your error message understand that this error is occuring due to Missing of Number assignment account group to customer at config level.
Pls check its assigned or not, if not pls try to assign, below configuration path may helps you.
Spro-Flexible Real Estate Management (RE-FX) -> Business Partner -> Relevant Settings for Business Partner in RE Context --> Business Partner - Customer --> Number Assignment: Assign Account Group to Customer-Exicute
Here you have to assign the grouping to account group.
Then check the FLBPD2 transaction, it may works, Pls revert with status if still error persists.
Regards,
Subbarao.Narne