Hyde here from Shopify.
If you could provide a little more information about the context, that would greatly help me help you!
Some of the questions I have for you are:
1) Is it just the one customer?
2) Do you have a link to the product they're trying to buy?
3) Have you completed the set up of the product as per our guide?
4) What error message are they receiving when trying to make the purchase?
5) What payment gateway are they using?
6) Is there any feedback from the gateway in the timeline of the abandoned checkout?
I look forward to hearing a little more detail about the situation so I can dig into it with you.
All the best, Hyde.