Hank here from Shopify.
That message can come from a wide range of issues. The most common are down to issues relating to special characters in your store name, or your stores physical address not being 100% confirmed.
As you mentioned you were using PayPal to purchase this, it could also be a case that there was an issue on PayPal's end, so I would advise just confirming with them if there was any particular issue with the purchasing of the domain.
After that is done, can you ensure that your address, etc in *Settings > General* of your admin is 100% accurate, and try to purchase the domain once more?
Let me know how you get on, or if you receive an error, can you take a screenshot to share with me and I will gladly assist you further!
All the best,