You may want to try an incognito mode session, or a different browser. I’ve seen issues before where PayPal fails to allow you to log in due to some combination of cookies stored on your browser.
Thanks for the tip. That worked and I was able to get past the login, but then I got an "unprocessable_entity" error. There might be something wrong the API?
EDIT: Disregard the above. I just tried it in another browser and that worked perfectly. My PayPal account is now linked successfully. Thanks for your help!