If you use Google search, you will find this information on how to import a SKP model into Softplan.
Because Softplan 2020 is currently importing SketchUp 2018 files
Import and Export support added for SketchUp 2018 file format.
Surfaces exported to SketchUp are logically grouped to make modification easier.” https://softplan.com/?page_id=5413
before you try to import the 3D model, you just have to be careful and download this type of file from the 3dwarehouse - click on
Download > SketchUp 2018 Model
or, if you save from your SketchUp 2019, to use
File > Save as > Save as type as 2018 version.