Trying to issue a refund to an invoice in our sandbox. It appears all invoices have this issue.
Sandbox app id: sandbox-sq0idb-9OyqW_2Obzu1AeWT2tbAIw
Maybe a CORS issue since the request is forwarding to the main squareup.com domain:
Thank you
Are you trying to refund the payment in the sandbox seller Dashboard? If so that is a known limitation in Sandbox. Payments in sandbox can only be refunded by the API at this time. 
Yes, attempting through the Dashboard UI.
Thank you for the quick response!
Is there a list of known limitations or issues anywhere?
Yes, our Sandbox limitations section of our docs has all the known limitations.