It could be the matter reference is invalid, or not being entered in the correct format. Try entering the matters case where you can then select the Ledger Card. You can enter a matters case by following the steps below:
From the DPS One Office/FD home screen select File.
Select Matters.
Search for matter by entering the Matter Ref, or click on Magnifying glass and enter matter details then click on the search magnifying glass.
Once matter is loaded select OK.
Click Ledger Card.