Shadows are an integral part of the real 3D world for Lumion. 

For minimizing shadows, try these settings: 

• Overcast Style: Optional but provides for a cloudy sky with mostly ambient light.  Or use the Sky and Clouds Effect with an overcast sky.
• Sun Effect: Make sure this Effect is above the Real Skies Effect in the Effect Stack and then set the Sun Brightness slider to 0.
• Sky Light Effect: Off.
• Shadow Effect: Set the OmniShadows slider to 0. 
• Shadow Effect: Set the Fine Detail Shadows slider to 0. 
• Adjust the Exposure Effect to suit.

The shadows are still there but they are now very lightly colored because the sunlight is very diffuse.