I’m also getting this:
Assets\AwesomeTechnologies\VegetationStudioPro\Runtime\Utility\BVHTree\BVHBBox.cs(3,13): error CS0234: The type or namespace name ‘Mathematics’ does not exist in the namespace ‘Unity’ (are you missing an assembly reference?)
This happened after importing Vegetation Studio.
My Unity version is 2019.1.14f1