I'm using Unity 2020.3 LTS and when importing the Mapbox package, if you don't need AR tools, you can remove the GoogleARCore, MapboxAR, and UnityARInterface folders and it works fine.

1. Download Package From here and import.
2. In unity, go Mapbox menu item, and past your map box key/token.
3. Open an example see from Mapbox > Examples and play

Congrats! MapBox installation has completed. For more details info visit offical docs.

New versions of Unity (2019.4 and 2020) are NOT supported. If you download the package you get all kinds of errors.