This article explains the workaround solution for failed Graceful switchover with the below-mentioned error code
Graceful Switchover failed due to activated Unified auditing
Applicable Error Code
ERROR_CODE:600 ', 'ORA-00600: internal error code, arguments: , , , , , , , , , , ', ',  (DBD ERROR: OCIStmtExecute) '
Standby v9 to 9.0.16.
Note: this issue is fixed in 9.0.18
Workaround Solution :
1. Disable be Unified auditing before switchover. Refer to the below mentioned MOS notes for reference
2. Try Switchover again
dbvctl -d <DDC> -o switchover