ORA-01156: recovery or flashback in progress may need access to files

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SQL> alter database drop standby logfile group 11;
alter database drop standby logfile group 11
ERROR at line 1:

ORA-01156: recovery or flashback in progress may need access to files

Solution: cancel the recovery. delete the logfile group and start the recovery again.
SQL> recover managed standby database cancel;
Media recovery complete.
SQL> alter database drop standby logfile group 11;
Database altered.
SQL>  recover managed standby database disconnect from session;
Media recovery complete.