Are you able to sign in to your PayPal account directly? Are there any messages there or indications of things you need to do such as update a payment method?
If you are having problems signing in to PayPal when checking out on eBay, sometimes it is worth trying a different browser, or trying to check out using the eBay app.
You may be able to get around some PayPal sign-in issues by enabling auto-login in your PayPal account settings for your browser, but you should only do that if you are the only one that has direct access to your computer.
