The initial treatment date is entered on the add / edit client form in the Advanced area in the date box labeled First Treatment Date. The payer is rejecting, stating that the initial (first) treatment date you have put on this claim is after the date of service for the service line that you have put on this claim.
How to Fix
Open up the edit client form for this client/patient and click the Advanced button on the edit client form. On the right, change the date you have entered in the initial treatment date box. Remember to save the client.