Cause

No part has been assigned to the part placement.

Solution

1. From the Message management dialog, double click the message to locate the relevant part in the layout space.
   
   In the layout space the 3D graphic of the part placement is highlighted in color and enlarged.
2. Double click the 3D graphic of the part placement and enter the missing number in the Properties <...> dialog of the part placement, Part field.
3. If required, start a new check run.