If you do not customize the sample text on the card(s) in your mailing, you will see this warning message: "Please go back and edit the sample text on your card(s) before proceeding."
Once prompted with this warning message, you will need to click on the left black arrow (see below) to navigate back to your main card to edit the text. If you have an insert card, you will need to edit the text on that as well.
After all your card text is updated, you can click on "Next" to save your changes and move on. Please make sure the design tab is only open in one browser window/tab at a time so that changes save properly.