Error Message: ORA-01244

and below are steps to resolve the problem.

First thing to do query v$datafile to check the unnamed datafile with this query:

select name from v$datafile;

after get the information, set the file management to Manual on Dataguard with following query:

alter system set standby_file_management=manual;

and then issue the following command  to recreate the missing datafile:

alter database create datafile '<path from first step>' AS '<path from missing datafile that can get from alert.log>

wait for awhile to finish the process, it really depends on your server capability and size of the missing datafiles. If  it is done then set dataguard file management auto

alter system set standby_file_management=auto;

then run the recovery mode as usual:

alter database recover managed standby database disconnect;