Ted here from Shopify, thanks for reaching out to us!
What type of plan have you selected for your store? If you are on a trial, dormant or a development plan, it's not possible to receive or test orders at the checkout until a plan has been selected. Our price plans can be found here and more info regarding plan features can be found in this guide if you would like to take a look. In addition, if you are on a trial, you won't be charged for anything until your free trial fully ends and a great breakdown of how your 30-day invoice might appear can be found here too.
If you have already selected a plan, would you be able to take a screenshot of the error for me? Who is your chosen payment gateway?
If there is anything else I can help you with, please let me know, I'm happy to help!