ORA-39701: database must be mounted EXCLUSIVE for UPGRADE or DOWNGRADE

I am trying to start the database in upgrade mode and received the error as below:

SQL> startup upgrade
ORACLE instance started.
Total System Global Area 3758096384 bytes
Fixed Size                  4503952 bytes
Variable Size            3103786608 bytes
Database Buffers          637534208 bytes
Redo Buffers               12271616 bytes
Database mounted.
ORA-01092: ORACLE instance terminated. Disconnection forced
ORA-39701: database must be mounted EXCLUSIVE for UPGRADE or DOWNGRADE
Process ID: 10523
Session ID: 2092 Serial number: 36861

SQL>

Cause:

The reason for getting this error message in my case was that I was trying to issue this command on RAC database which had two instances.

SQL> show parameter cluster_database
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cluster_database                     boolean     TRUE
cluster_database_instances           integer     2
SQL>

Fix:

Just turn the database clustering off and try starting it again in upgrade mode

Start database as normal and alter the parameter as below:

SQL> alter system set cluster_database=FALSE scope=spfile sid='*' ;
System altered.
SQL>

Now shutdown the database and then try the startup upgrade

SQL> startup upgrade
ORACLE instance started.
Total System Global Area 3758096384 bytes
Fixed Size                  4503952 bytes
Variable Size            3003123312 bytes
Database Buffers          738197504 bytes
Redo Buffers               12271616 bytes
Database mounted.
Database opened.
SQL>