When does this error pop up? When you're trying to pay for the DLC here on itch.io? Worst case, you can actually get the DLCs on Steam, and they will work for the itch version as well. But hopefully we can figure something out here.
Sorry for the lateness of this reply! Unfortunately, looking at this image, it appears to be some kind of error with PayPal. I don't think there's anything we can do to help with that besides suggesting another payment method, if possible. So sorry about that!
