Your eBay account may not be linked to your PayPal account. To check, follow these steps:
If your eBay and PayPal accounts are already linked, you may have entered an incorrect email address when creating your listing. You can check by selecting Revise in your listing and scrolling down to the payments section.
If your eBay and PayPal accounts are linked, and you entered the address correctly, you'll need to contact PayPal for further assistance.