Hank here from Shopify.
Sorry to hear you are having some difficulty in purchasing your domain.
There are a few reasons this might be happening, and I'll try to list some of the most common reasons below.
The first step would be to ensure there are no special characters (á, ń, ö, etc.) in your name, address, etc, in Settings > General, as this information will be used to register your domain and will need to be validated. Special characters are not compatible with domain languages, which leads to a common error.
Another error may be insufficient funds or if using the likes of PayPal, I'd advise you to see that your account is not showing any issues, or timeline errors (failed payments, etc).
The last potential issue is if trying to purchase a premium domain (usually a short worded URL), these types of domains are not compatible with Shopify, and will always give an error message.
Would you be able to share a screenshot of the error and I will try to see if I can spot any issue with the steps you are trying, or if I can see any potential reason as to why this is not happening for you?
Let me know,
Thank you for your reply.
However, I did not see any images attached to your response. Can you try once more using the rich text editor above your message?