Open Api resource "Loading spec.."

Hi! We want to use Open Api for our resource, we can connect the URL but when we try to use it we always got stuck with the message "Loading spec..." Maybe there is a problem with our .json code. How can we know a more specific error about what is happening?
I attached a picture of the error and the .json link.

Error:

Test Connection:

Open Api URL: https://docs-test.letsbit.io/payments/public_api.json