A payment with Paypal may be declined for several reasons. If your payment was declined, you may need to:

Confirm the email address associated with PayPal.

Update the expiration date or billing address associated with your credit or debit card.

Confirm your credit or debit card so we know you’re the card owner.

Go through a security check so we can confirm your identity.

Resolve a limitation placed on your account.

If none of the above apply or if you continue having issues with your payment, you can also try:

Changing your payment method at checkout. If you only have 1 payment method linked to PayPal, you’ll first need to add another payment method. You can add a debit or credit card, or add a bank account. 

Contacting your card company or bank for more information. To protect your privacy, your card issuer or bank doesn't tell PayPal why your card was declined. If you’re still unable to make a payment, contact them directly for details.  

More details can be found here: https://www.paypal.com/uk/smarthelp/article/why-was-my-payment-declined-faq3039