Assuming it was a real sale, and was cancelled after the sale went through, the most logical reason was due to credit card fraud. The card holder gets a an alert on their card, checks it, and reports it as fraudulent. The card company then yanks the money back from Etsy.
Etsy is not going to tell you what the issue was as it would be a privacy violation. Etsy likely doesn't even know the real reason. But just knowing it was cancelled is enough.
You should be aware the scammers and thieves always target new shops. They figure new sellers will be naive and excited for their first sale and ignore the red flags. They will pretend to be a buyer, they will pretend to be Etsy, they will tell you lies about how Etsy operates and claim you need to update your bank info. They will even direct you to faked web pages that look like Etsy in order to steal your money. Don't fall for it.
