You may have already tried registering for that category. Click on the “View Cart” button to see what you already have in your shopping cart for payment. If that registration is correct, you may continue. If it is not, you can edit it or delete the entry altogether by clicking on the appropriate button on the payment summary.
If you were trying to register another person when getting that message, click on the “View Cart” button and review the registration. Choose the appropriate button below the payment summary to register for another category (within the same event), register someone else, register a minor, or register for another event. Then you may continue the registration process.
If you are still having trouble you can close your browser and delete cookies in order to clear session content and start the registration again.