The "sis_user_id" is a field that seems to only be in the response of the API and not one of the parameters that the call accepts when sending the request.
Try changing it to just 'user_id' and try again. The data being sent should look something like this with the user_id being an integer:
{"enrollment": {"user_id": 1234567890,"type":"TeacherEnrollment"}}
I'm not sure how your environment is set up, but the user_id is the ID that Canvas assigns a person (and is listed in the URL of the user's profile) and not what the actual "sis_user_id" may be. In our instance, they are different.
Hope this helps! Good luck!