ORA-16006: audit_trail destination incompatible with database open mode on startup standby database

ORA-16006:	audit_trail destination incompatible with database open mode
Cause:	The audit_trail initialization parameter was set to "DB" (or TRUE), which is incompatible with a database opened for read-only access.
Action:	When the database is opened for read-only access, the audit_trail initialization parameter can only be set to "OS" or "NONE" (FALSE).

Reason: audit trail parameter should be in OS or NONE when standby database configured.
Solution: change the audit trail parameter value to OS and NONE. and start the standby database.