Ted here from Shopify, thanks for posting your query!
Would you be able to share the domain you're hoping to buy with me here? I'm happy to take a closer look into this with you! If you would prefer not to share the domain with me publicly, our support is available here 24/7 as well.
Happy to help!
That's perfect thank you!
It looks like your domain name is fine, however, it's likely that your billing address has a special character listed and our domain registrar can't process that character. If you log into your Shopify account and click Settings > Billing, can you replace the wording with a different character? For example, if "ó" is listed please use "o" instead. After that, please try to buy the domain again. You can always change the billing address back to the original address after the domain has been processed.
Let me know how this goes and if there is anything else I can help you with, I'm happy to help!