Why does the model tester return “Failed to call Model”? 

Issue: The model tester can present with "failed to call model".

The reasons why it occurs and the resolution: 

1. The URL routes are not generated properly, this can happen if the deployment timed out/stalled for a while. Go to Settings/Deployment and click the "Regenerate Routes" button, retry the model tester again.
2. The request is malformed (bad JSON) - This is quite often the culprit - try using an online json syntax checker to identify, rectify and then re-try.
3. There is a SSL certificate mismatch (if you get a "General SSL Engine" 500 error, this is likely the culprit). If using curl you can hit the model, then this is definitely the issue. There's no easy solution for this, you will have to get your certificate and update the cert chain. Please contact Domino support as we will be able to assist with this.
4. A floating point number (float 32) is being returned as a result in your code, this will be typically seen in the instance logs as: TypeError: Object of type float32 is not JSON serializable The resolution in this case would be to amend your code to return an integer instead of a floating point number.
5. When you use 'single quotes' instead of "double quotes" in your JSON. JSON only allows "double quotes" (unless you control the data, and can use a library that is more liberal at parsing JSON)

Ultimately, your first point of call is to always look at your model instance logs: select your model > versions > select Instance log.