Paypal balance payments are not able to be used for subscription payments unless you have a credit card or bank linked to your Paypal account. This is a Paypal policy, they don't allow you to pay for recurring billing services unless you have a backup payment method linked.
If you want to pay with your Paypal balance, you will need to buy a plan that doesn't have auto-billing enabled immediately. You can do this by using one of the links below, they are the same plans we normally offer on our site but without auto-billing enabled:
1 Month = https://windscribe.com/upgrade&promo=paypal01
1 Year = https://windscribe.com/upgrade&promo=paypal12