Hi @bkuss, thanks for your patience whilst I tried to reproduce the issue. I think perhaps this may be an issue with Core SDK now being 3 years old and that a recent update to VS2019 .Net Core libraries has caused now an incompatibility throwing this namespace error.
Unfortunately, the Core SDK is no longer maintained and we have diverted resources into its replacement the Tobii Interaction Engine which you can check out @ https://developer.tobii.com/consumer-eye-trackers-interaction-library-getting-started/
We also have the Tobii Stream Engine API which offers a reduced footprint low-level access to eye-tracking data available @ https://developer.tobii.com/consumer-eye-trackers/stream-engine/getting-started/
You had not mentioned if you were developing from scratch or porting an existing project. Hopefully it is the former, in which case we would advise that you migrate to our newer and maintained SDK platforms.
If you would care to describe in more detail your circumstance, I would be happy to try and suggest the most appropriate course of action. Thanks for your understanding.