I have Sketchup Pro version 2016 and am no longer able to download models from 3D warehouse. The models are only accessible for versions 2017 and above, some only for 2018 and above.
Why did Sketchup stop allowing earlier versions to access Warehouse models?
The 3D Warehouse supports the latest three versions of SketchUp. With the release of SketchUp 2019, that means only 2017 and newer. You can download the Collada file for the model/component you want to use via your internet browser and import that into SU2016.
There are a number of threads on this topic already and the reasons for support being as it is have been given.
If you use the Components panel, and click on the thumbnail of something you want, it will load into SketchUp 2016.