Authentication Failed - Sandbox

[SS_2019]

We are currently trying to implement Seller onboarding using the PayPal docs however when we attempt to send the JSON request via JavaScript we are receiving the following error:

{"name":"AUTHENTICATION_FAILURE","message":"Authentication failed due to invalid authentication credentials or a missing Authorization header.","links":[{"href":"https://developer.paypal.com/docs/api/overview/#error","rel":"information_link"}]}

 The auth header is being sent through as such:

xhr.setRequestHeader("Authorization", "Bearer OUR-ACCESS-TOKEN");

This token was generated via the steps on this docs page. We also tested it using the example shown here. This returned:

{total_count: 0, invoices: Array(0), links: Array(1)}

I'm assuming there can not be an error with the Authorization header line if it works for the prior. But then if that is the case I do not understand why this error is being recieved. 

Full JS:

var xhr = new XMLHttpRequest();
var url = "https://api.sandbox.paypal.com/v1/customer/partner-referrals";
xhr.open("GET", url, true);
xhr.setRequestHeader("Content-Type", "application/json");
xhr.setRequestHeader("Authorization", "Bearer OUR-ACCESS-TOKEN");
xhr.onreadystatechange = function () {
   if (xhr.readyState === 4 && xhr.status === 200) {
     var json = JSON.parse(xhr.responseText);
     console.log(json);
   }
};
xhr.send(data);