Hi Florian,
SAM's integration with Python for this implementation of the LandBOSSE model is somewhat experimental and may not work properly on all operating systems. I have created an issue for this, but am not sure we will have the resources to address it for the next update to SAM:
github.com/NREL/SAM/issues/1420
In the meantime, you might try installing the the model from its GitHub repository at
github.com/WISDEM/LandBOSSE.
You can contact the repository contributors if you have questions about that implementation.
Best regards,
Paul.