Atrium relies on a User's Role History to track each user's Discipline & Segment assignment and their place in the org chart.
To correctly attribute a user's data to their manager's team totals and averages, Atrium needs an accurate record of who that user reported to throughout their tenure with your organization. Thus, Atrium requires the "Manager" assigned to each of the user's role history records to have been a manager for the entire period they are assigned to.
If you have set the manager to "N/A" deliberately, that's fine. This just means that for the period in which the user's manager is "N/A," the user's data will not be included in any manager's team totals or averages, and will not "roll up" to totals and averages across the organization.
There is also a case in which Atrium will assign "N/A" as the manager automatically:
When a user is created for the first time--either during your initial Atrium implementation or via the org chart editor--their first Role History Record is created with their initial start date and current Discipline, Segment, and Manager. Atrium then validates the start date of the user and their associated manager to ensure the manager started at the same time or earlier than the user. If that manager started AFTER the user, Atrium will insert a Role History Record for the time period between the user's start date and their current manager's start date and assign "N/A" as the manager.
To update that initial Role History Record with their correct manager, you can "Edit" the user's Role History and update the manager field.