PayPal Login issue Work around.
Sometimes PayPal login not works even after you entered the correct credentials. It is a known intermittent issue from PayPal. Please try to buy as a guest option in PayPal login screen (bottom part of the screen) and enter your details then proceed to purchase using your card. Below is the screenshots of this known issue.
If buy as guest option is not working, please ensure you have your credit/debit card attached to your paypal account.
You can't use funds in your paypal account balance if you don't have a back up card linked to paypal as well.
Please refer this guide on how to link your credit/debit card to your PayPal account.