It seems like there might be a problem with your card or with your PayPal account. Unfortunately, since PayPal handles all of the payments, we don't know why the payments aren't working. However, you can try contacting them at 402-935-7733 or by emailing them and they should be able to help you out.
Another alternative which may work better is purchasing a non-recurring membership from the following page: https://www.lingq.com/accounts/choose-products/
One time payments typically work fine, even when standard recurring payments are rejected.