Error when trying to drop an undo datafile

DBAs,

The environment is 10.2.0.3 RAC.
When I added an undo datafile to the undo tablspace for one instance in a RAC environment.
Due to the mistype of the datafile name, I wanted to drop the datafile.

I used:

alter database datafile '/CODSP/vol6/und/undotbs6_06.dbf' offline for drop;

The undo file is offline. The command was run without error.

Then I used trying to drop the datafile--
alter tablespace undotbs2 drop datafile '/CODSP/vol6/und/undotbs6_06.dbf';

I got the error:
ORA-03264: cannot drop offline datafile of locally managed tablespace

So I wanted to put that datafile online:

The Oracle error says the file needs media recovery.

Currently, the status of this file is recover.

So, at this stage, what should I do?

Thanks,

Lily

recover the file