If SwitchList's car movement algorithms cannot figure out how to direct cars to their destinations, SwitchList prints helpful error messages in the Problems Window. This window only appears when problems are found.
You don't need to fix every one of these errors; they simply indicate that some cars will not be moved because destinations cannot be reached, and some industries will not receive traffic. You can still print switchlists, change details on your layout, and use SwitchList when problems appear.
Difficulties in Car Movement for a list of problems that can appear in the window, and the common solutions.
Detail: Moving Cars Only If Space At Industry to understand why a freight car will not be moved to an overflowing siding.