I set up a new intervention group which started in September but can't add data for September, why is this?

In Workspace>Interventions, first check the Start Date for this Intervention Group, then click on Edit Data to see the date the students joined this group. You may need to amend the student start date to the Group's start date. Click on a date in the Joined Group column in order to edit it. You will then be able to input data from the Group's start date.