This is the most common problem. If you load the map and see nothing but a blue sky with no buildings or roads, it means the map is missing required (like buildings or street signs) or splines (the 3D curves that form the roads). To fix this, you need to identify what is missing.
Ultimate Guide to OMSI 2 Warszawa Wilanów: Download, Installation, and Review Omsi 2 Warszawa Wilanow Download