This is not necessarily an error, it is just indicating that a part that is in your CAD assembly and cBOM isn't currently in your eBOM. This could be done intentionally for things like reference geometry or consumables.
You can add these parts back to your eBOM by checking out and checking back in the assembly. The BOM anomaly window will be displayed where you can add the items back to the eBOM structure by changing the red x next to the parts to a green check mark. You can read more about using the BOM anomaly window in this article.
https://help.autodesk.com/view/UPCHN/ENU/?guid=UC_MechanicalCAD_mc_cad_plugin_mc_bom_anomalies_html
