Hi @me-supachai and thanks for your query. Indeed, it should be quite possible for you to use the Stream Engine within Unity. First thing to report to us is which .Net version you are using? It may be that you are using .Net 3.5 whilst the Stream Engine Wrapper dll is .net 4.5
That being the case, you may be able to use this Stream Engine Wrapper @ https://www.nuget.org/packages/Tobii.StreamEngine/
It would also help if you could kindly provide as much detail as possible in your reply with respect to your system, IDE & Unity versions for us to best diagnose the issue.
Also, are you able to run the regular Stream Engine API sample without Unity, ie in a regular VS session? Thanks for the clarification.