YouCaring donations are processed by WePay or PayPal, depending on which payment processor was selected by the fundraiser organizer. Both of these providers have different requirements at the checkout screen.
One of the most common causes of credit card rejections is a mismatch between the billing address on the card and the information entered at the payment screen.
If you are receiving an error at the checkout step, you should double check the Billing Address, Name, Expiration and CVV on your credit card to make sure they match what you have entered.
If this doesn't work, you may consider trying another credit card, or reaching out to the fundraiser directly using the 'Contact the Organizer' button to see if you can send a direct donation.