Hei, I too faced the same issue. In my case, I was using a personal account as a developer account. When I created sandbox credentials from the Business account it started to work. So make sure, 1. Developer account is linked/created with the business account 2. Sandbox account is created from that. 3. Sandbox account should also be a business account and it should be enabled for advanced payments ( Using this link: https://developer.paypal.com/docs/business/checkout/advanced-card-payments/#enable-your-account )
Also be sure to not include "disable-funding=card" in the javascript SDK parameters. I was upgrading from standard payments where I had this option enabled and this forced isEligible to always return false even if my account was approved.
