Why can't I issue a full refund to a customer via PayPal?

I’ve been attempting to issue a full refund to a customer, but I’ve encountered an error message that says, “Unable to refund €23.92 EUR.” The customer originally made their payment through PayPal. Afterward, we transferred that payment from PayPal to our bank account. Now, when I attempt to issue the refund through my Shopify store, it won’t allow me. Could someone please advise on what might be causing this issue?