Thank you for the screenshot of error message you are seeing at the checkout. While this is a common thing to have happen, especially on new store, tracking down the source of the issue can take a bit of work. I shared some of the most likely causes for this issue in another thread which I will link below. I will also provide a copy of my original message in that thread with the next best steps for you to have a look at.
This can be a bit tricky to solve as there are a lot of things that can create this issue. I'll list some of the most common things that can cause this and you should hopefully be able to get it resolved quickly!
If you are still not able to find the issue please don't hesitate to reach out to our live support for additional help with this. To contact our live support please follow this link: Shopify Help Center - Contact Support, sign into your store account, search for your issue and use the contact support button at the bottom of the search results to see all our live support options.