I'm working on a data migration project that requires web service keys to be created for each student.

The main migration app's api token is used to assign the role to users with a URL parameter like:  
wsfunction=core_role_assign_roles&assignments[0][contextid]=1&assignments[0][contextlevel]=system&assignments[0][roleid]=%s&assignments[0][userid]=%s
 
where roleid=10 is the student migration role to grant those users permission to use the quiz API and the ability to make webservice tokens.

I was able to assign the role to users with the web service API above successfully, then was able to make a user token using the Profile > Preferences > Security Keys page.
 
I then tried to verify the key in the admin's Web Services > Manage Tokens screen, then was presented with an error:

Exception - core_webservice\reportbuilder\local\systemreports\tokens::get_missing_capabilities(): Argument #3 ($serviceshortname) must be of type string, null given, called in [dirroot]/webservice/classes/reportbuilder/local/systemreports/tokens.php on line 146