ORA-01507
ORA-01507 means that the command you want to execute cannot be done at NOMOUNT state, you should mount the database from NOMOUNT for further operations.
Let's see some error patterns of ORA-01507.
Let's see those cases.
Alter Database Open
To change the state of a database from NOMOUNT to OPEN, you can't make it in one step.
SQL> alter database open;
alter database open
*
ERROR at line 1:
ORA-01507: database not mounted
Instead, you should do it in two steps.
SQL> alter database mount;
Database altered.
SQL> alter database open;
Database altered.
Which means, the solution is to mount the database first, then open it.
RMAN Commands
Tried to run some RMAN commands under NOMOUNT state, but they failed with ORA-01507.
RMAN> list backup;
...
ORA-01507: database not mounted
RMAN> configure controlfile autobackup on;
...
ORA-01507: database not mounted
The error message told us that the database is now NOMOUNT, there's no way to retrieve information from the control file.
To solve ORA-01507, you need to open it to MOUNT state.
SQL> alter database mount;
Database altered.
Then issue your RMAN commands again.
Shutdown Immediate
In the midway of database shutdown, you may see the error, too.
SQL> shutdown immediate;
ORA-01507: database not mounted
...
The error message displayed at this moment is just a notification, NOT an error. It tells us that the database now is dismounting, so you don't have to worry about it.