RMAN-06403: Could not obtain a fully authorized session tips 

RMAN-03002: failure of allocate command at 01/13/2015 04:27:08

RMAN-06403: could not obtain a fully authorized session
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory Additional information: 3640 Additional information: 1158587339

SOLUTION:

One, you are trying to work with a wrong database which does not exist. Second, you want to work with the correct database only, but it has not started yet. Looking further, your doubt turns into confirmation as the next message is ORA-01034 saying that Oracle is not available which means that your database is not yet started. 

So, Please check that you are giving proper DB SID without caps or mistakes.