If there are errors executing a REST API call, you can check the OAS Configuration app, click on the triangle icon (should be flashing of there's an error) and you'll see system errors.

Locate the REST API and expand it out and you should see the failed calls. But it often won't give you the detailed HTTP request/response and just contain the failed URL that was attempted. It's a good way to see if people are hitting incorrect endpoints.

If you use Postman to execute REST API calls, you can see the proper URLs, headers, body contents, etc. for making proper calls.

If you ever get these response or codes, this is what they mean:
401: Unauthorized - you have not included the clientid and token fields in the request header, or the session has expired
500: Unknown server error - this may be something we need to investigate since the data was submitted properly but an error occurred on the server processing the request.
404: The object you're trying to GET or PUT (update) does not exist

If you see "Service Unavailable" that means the REST API did not start up properly.
If you see a message indicating the Endpoint does not exist, this means the URL is not correct for the call.

If there's something specific you're attempting and don't know what the issue is, you can always look at a successful call from Postman's console and it will expose everything in the request header/body and response header/body. You can compare it to your failing call to see what you might need, such as the correct Content-type. You can always let us know what call you're stuck on and we can investigate why it might not work for you.