It looks like some people in the thread below have figured it out — we actually have this. You can start a chat both ways; the system will automatically recognize if this person is on Skype for Business or on Teams.
Thanks and let me know if you have any questions.
Please fix: This user is currently using Skype for Business. Some Microsoft Teams features, including chat history won’t be available. I had the same problem. Had to set up a different channel to chat. It would never let me get through to that person again! Keep getting the above error message.