As soon as you have sent an Invoice to your client, you need to mark it as Sent. Payments can be recorded only when an Invoice has been marked as Sent. Once you have recorded all of the payments that amount to the total value of your invoice, the Invoice will be marked as Paid automatically.
Go to Invoices in your navigation bar.
Click on your Invoice and change its status to Sent. Add the correct date and click Update sent date. Your invoice is now Unpaid.