Cause

No part has been assigned to the part placement.

Solution

1. From the Message management dialog, double click on 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 on the 3D graphic of the part placement and enter the missing number in the Properties <...> dialog for the part placement, Part field.
3. Then start a new check run.