ORA-01586: database must be mounted EXCLUSIVE and not open for this operation

 You can choose among multiple options to drop Oracle databases. 

1. DBCA (Silent Method or GUI)
2. Command Line Interface (startup mount restrict and then issue drop database) using sqlplus or rman
3. Manually cleaning up files

SQL> drop database;
drop database
*
ERROR at line 1:
ORA-01586: database must be mounted EXCLUSIVE and not open for this operation

We are decommissioning the RAC database here. So we need to change the parameter cluster_database=FALSE.

Set your database profile.

srvctl stop database -d <DB_name>
SQL> startup mount;
SQL> alter system enable restricted session;
SQL> alter system set cluster_database=FALSE scope=spfile;
SQL>Drop Database;
Database dropped.