This issue often occurs at Oracle database 12c installation process. But I faced with that error message during restore/recovery operation…

This error occurs whenever we want to mount the database but the multitenant container database was not set up properly. To fix this, follow below commands

1. Connect to your Oracle 12c using the sysdba user and run a startup nomount command on the SQLPlus command line interface:

SQL> sqlplus / as sysdba

SQL> startup nomount

2. Run below

ALTER SYSTEM SET “enable_pluggable_database”= TRUE SCOPE = SPFILE;

3. Now, shutdown&startup database. You will see now you can mount the database:

SQL> shutdown immediate
SQL> startup mount;

PS: You need to put that parameter on init file for restore operations. Otherwise you can not mount database.