When attempting to generate a Structural Quantity Spreadsheet I am getting a "could not find part" error and the results are incorrect.
The error message in this case is a literal description of the issue – certain structural members in the DGN file have parts assigned that were not found at the time the spreadsheet was generated. You can use the Verify Parts tool to help troubleshoot since any missing parts (custom in this case) are displayed in red. You can then modify the applicable Structural members by reassigning them parts that do exist and then run the Structural Quantity Spreadsheet again.
See also: https://docs.bentley.com/LiveContent/web/OpenBuildings%20Designer%20Help-v2/en/VerifyPartsDbox.html