Cause
Possibility 1
A part placement in the layout space disposes of a specific part number. No part number or a different part number was on the contrary assigned to the associated main function / main terminal corresponding to the part placement.
Possibility 2
A part placement that disposes of a specific part number was inserted several times on a layout space. The associated main function / main terminal on the schematic page corresponding to the part placement disposes of the same part number. However, it has a lower value with regard to the property Number of units / quantity at the part than the number of part placements inserted on the layout space.
Solution
Regarding option 1
Enter the same part number at the part placement in the layout space and at the main function in the schematic.
Regarding option 2
Adapt the entered value for the number of units / quantity in the part properties of the main function / main terminal in the schematic to the number of the part placement inserted in the layout space. This means, either increase the value for the number of units / quantity at the main function / main terminal, or delete the surplus part placements.
If required, start a new check run.