Why can't I edit the "Is a manager" attribute for a user? 

The attribute "Is a manager" is not an attribute that can be edited directly on users in the organisational structure. 

This attribute displayed as "Yes" or "No" is based on however a user is selected as the manager of one or several groups in the organisational structure's group structure. 

If you edit a group and select a specific user as its manager and save, the user will then have "Yes" as the value for the attribute "Is a manager".