ORA-01157: cannot identify/lock data file

The data file is missing or corrupt.

Solution: Restore from backup but if you don’t have a backup, you can’t restore and recover it and of course you  can’t restart your database. So, you have take the data file offline before starting the database.

Example :

SQL> alter database datafile '/u01/data/19c/data01.dbf' offline for drop;

And open the database:

SQL> alter database open;

And when you run RMAN backup job:

SQL> alter database open;
RMAN-03002: failure of backup command at ..
RMAN-06056: could not access datafile 1

The solution for the above RMAN error is to exclude the offline data file from the backup.

RMAN> backup database skip offline;