You can try the following steps to mitigate your error:
1. Examine the Appian logs: Examine the Appian logs for any errors that occurred on the server. This will help you understand what went wrong and why.
2. It could be a problem with access rights. Please verify that the user has at least viewer rights to the API-related objects such as datastore, etc.
3. Validate input parameters: Ensure that the input parameters are in the correct format and not causing any problems.
4. Verify connectivity: Ensure the connected system is accessible and operational.
5. Check authentication: Verify that the authentication method used in the integration is correctly configured and that the authentication token is still valid.
6. Increase the timeout: See if increasing the timeout in the integration configuration solves the problem.
7. Debug the integration: Use the Appian Debugging Tools to troubleshoot the integration and pinpoint the source of the problem.

You can try the following steps to mitigate your error:
1. Examine the Appian logs: Examine the Appian logs for any errors that occurred on the server. This will help you understand what went wrong and why.
2. It could be a problem with access rights. Please verify that the user has at least viewer rights to the API-related objects such as datastore, etc.
3. Validate input parameters: Ensure that the input parameters are in the correct format and not causing any problems.
4. Verify connectivity: Ensure the connected system is accessible and operational.
5. Check authentication: Verify that the authentication method used in the integration is correctly configured and that the authentication token is still valid.
6. Increase the timeout: See if increasing the timeout in the integration configuration solves the problem.
7. Debug the integration: Use the Appian Debugging Tools to troubleshoot the integration and pinpoint the source of the problem.

Hi Kapil,

I am facing similar issue, and tried out all the steps, but still the integration object is failing to give response, any other way to fix this issue?

 500 error comes at different situations like when no other error code is suitable or when system encountered an unexpected condition. Check integration logs (If you enabled them) to find root cause . If it's possible can you share the screenshot of the response.

Please have a look at Appian Community

 It's an Authorisation error and check whether the API accepts POST Method too.Just like Stefen suggested contact the API provider for changing the Status code to 401.