Activate user in APG

Why I recived this email: 

Dear Process Governance administrator,

Human task 'Evaluate change request ' was originally assigned to the following executors:
        - reviewer-apg(USER)

Either the users are not active in Process Governance, or the user groups contain no active user.

note that If a human task is assigned an organizational element of the Person type, the user must be activated for process automation.

how to activate it ??