Make sure you follow the setup guide in the VegetationStudioPro/Documentation folder. 
It is needed to follow all the steps there in order to install and run Vegetation Stuidio Pro.

The requirements are. 
Unity 2018.2+ 

Scripting Runtime Version: .NET 4.x Equivalent
API Compability level: .NET 4.x 
Both on Player settings.

5 packages from the Package Manager
-Mathematics
-Jobs
-Burst
-Collections
-Post Processing 

All these components needs to be installed and set up in order to use the new Job system and Burst compiler Vegetation Studio Pro is based on. 

For full functionality a graphics card that supports ShaderModel 5.0 is needed. 
Updated documentation and guides are also available online at https://www.awesometech.no