Hi,
    I'm encountering an error regarding web services and I'm not sure if I'm missing something or if what I want to do is not allowed.  I have created a new web service with a few of the functions from core_users.  I'm access that service via REST successfully, and I can create users, delete users, and search for users.  This tells me my token authentication and data formats are correct. 
I didn't find the right solution from the Internet.
References: 
https://moodle.org/mod/forum/discuss.php?d=3578002d App Intro