When you set up a support, you need to select the type of support through the options "Translations" and "Rotations"
Generally you need to find a model that best represents what you are trying to create in the real world. There are several types of supports typically used in a static analysis system. Pinned, Roller and Fixed (incaste) are the most common.
Truss models are modelled to be pinned. They are not meant to develop moments (otherwise its not really a truss). I have attached the three main support types you will use in an image:
I would suggest that you make the top support on your truss (the node on the upper left) as a roller and and the bottom support (the node on the bottom left) as pinned.
Najna I think the reason you don't have compression or tension in the karamba model member between the supports is that you need to set up the translations. They both appear to translate along the members axis. The top member needs to be a roller and the bottom member should be pinned.
Bottom should be TxTyTz and Top should be TyTz. I think that should fix the issue.