It is possible to use the PayPal payment option for other purchases without a credit or debit card added, however, you must have a verified back-up source of funds when you have only a bank account tied to the PayPal account. This is because PayPal only accepts instant payments for digital goods, and using a bank account via PayPal (paying with an e-check) is not considered an instant payment.
If you do not have a credit/debit card, you can simply add funds to your PayPal account that covers the amount of the purchase. Once you have a balance on your PayPal account, you will not be prompted to add a credit or debit card. If you do not have a balance in your PayPal account, you will always be asked for a backup funding source, ie: credit/debit card.
If you have any further questions regarding instant payments or any other issues with your PayPal payment, please contact PayPal Support directly: https://www.paypal.com/selfhelp/home
Something else to keep in mind before making your PayPal purchase attempt: Your PayPal account must be verified in order to complete purchases. Verifying your PayPal account protects you as the account holder, as well as the merchant.
For more information on what it means to have a verified PayPal account, as well has how to verify your account, please refer to the following page: https://www.paypal.com/us/webapps/mpp/security/verification-faq