How to debug the API Action process_mailing?
Harshal,

Check the resource url is set properly. Navigate to CiviCRM >> Administer >> System Settings >> Resource URLS. Click on '?' to see if all the variables have correct values and are pointing to right directories. If not then you may need to update the civicrm.settings.php file for $civicrm_root variable until the you get th variables right in resource url.

HTH

Pradeep