ORA-29913: error in executing ODCIEXTTABLEFETCH callout

Issue: When I try to access or view the records in my external table, I get below error. I get this error, when I run 'select * from <external_Table>' in my sql developer.

ORA-12801: error signaled in parallel query server P021, instance 2

ORA-29913: error in executing ODCIEXTTABLEFETCH callout

12801. 00000 -  "error signaled in parallel query server %s"

*Cause:    A parallel query server reached an exception condition.

*Action:   Check the following error message for the cause, and consult

           your error manual for the appropriate action.

*Comment:  This error can be turned off with event 10397, in which

           case the server's actual error is signaled instead.