Can someone please tell me what went wrong here .....
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SQL> alter tablespace psapundo
2 add datafile 'D:\UNDOFILE01' size 1m;

Tablespace altered.

SQL> alter database datafile 'D:\UNDOFILE01.DBF' offline;

Database altered.

SQL> alter database datafile 'D:\UNDOFILE01.DBF' online;
alter database datafile 'D:\UNDOFILE01.DBF' online
*
ERROR at line 1:
ORA-01157: cannot identify/lock data file 2 - see DBWR trace file
ORA-01110: data file 2: 'D:\UNDOFILE01.DBF'