Use

Before you can archive prepayment documents, the retention period must have expired.

The retention period is the period, which must have expired before you can archive application data.

You define the retention period for prepayment documents in Customizing under  SAP Utilities Tools Archiving Define Retention Period for Archiving Objects .

You can use Customizing from the archiving object (SARA transaction) to find all other Customizing activities for archiving.