Checkouts can be disabled because the form has reached the limit you set on the number of successful checkouts or because the form is past the date you set for it to be disabled. You can choose to display a custom message that your payers will see if they go to the checkout after it has become disabled.
1) Create a new form (or find the form you want to add a message to.)
2) Scroll to the Limit Checkouts section and click "Customize"
3) Type the message you would like to display. By default, we display "Checkout Unavailable" as the header and "This checkout is no longer available" under the header.
4) Click "Save" or "Save & Finish"