Hello @Firecracker, welcome to the Seller Community.
This is interesting, most declines come from the card issuer. In this case I recommend having your customer reach out to their bank or card issuer to inquire about the transaction. The bank doesn't share why a card was declined with Square.
If you were manually entering a payment and the information entered did not match what was on the card it will decline.
If you still have questions feel free to reach out. I'll keep an eye out for your reply. 👀
