stands for Direct3D 9 , an aging but still relevant API (Application Programming Interface) developed by Microsoft for rendering graphics. Even though modern Lumion versions rely heavily on DirectX 11 or 12 for high-end rendering, they often use legacy checks to probe your system capabilities upon startup.
If you're on a multi-monitor setup or recently changed resolutions, try launching your PC with a single monitor first. Run as Admin: lumion failed to create dummy d3d9