This error usually appears when you invoice for an item in the future. You are not able to invoice for something that has not yet occurred.

If this is accidental, please check the item dates within the invoice and adjust as required.

If you are wanting to receive payment in advance, you will need to take a deposit from the patient, and then invoice after the procedure/consultation has taken place. Here when choosing the payment type, you are able to apply the deposited amount.

Guide on adding a deposit is here.