Before an organization can take part in a test administration, the organization must indicate its participation in that administration.

From Setup > Organizations, select one or more organizations then open the task list and select Manage Participation.

Marking your organization as Participating means that you can create groups in that organization, assign tests to those groups, and assign students in those groups. Marking Participation Entry Complete, here or under Manage Completion Statuses, indicates its intent to participate in the test administration and finalized any data necessary to participate.

Manage Completion Statuses for Participation or Enrollment Counts for Multiple Organizations at Once

From Setup > Organizations > Manage Completion Statuses, view or change these settings: