My name is Lilith, I'm on the Social Care team at Shopify. Sorry to hear your new domain hasn't been connected yet.
Sounds like there might be something happening with your payment card or method. The error could also sometimes be due to the format of contact information attributed to your domain registration. Of course, there could be other reasons, but those two are the most common.
Nevertheless, I'd love to take a look at this for you. I've sent over an email to securely help you through our support channels.
I'm happy to answer any domain questions you think of as you work on your business.