The DBMS_SHARED_POOL package is not executed on the Existing Database, So the installation Cannot Proceed

Issue
You are installing EM Grid Control using existing database option and you
receive the following error:

The DBMS_SHARED_POOL package is not executed on the
Existing Database, So the installation Cannot Proceed.
Solution
If the DBMS_SHARED_POOL package has not been installed at the time of the
database creation, Oracle Universal Installer displays an error message and
prompts you to execute this package before proceeding with the installation.

To check whether or not the DBMS_SHARED_POOL package has been installed, login
to the database and execute the following query:

Select count(*) from dba_objects where OWNER = 'SYS'
AND object_name = 'DBMS_SHARED_POOL'
AND object_type IN ( 'PACKAGE','PACKAGE BODY') ;

This query should return a count of 2.

To install the DBMS_SHARED_POOL package, execute the following script:

$ORACLE_HOME/rdbms/admin/dbmspool.sql

REFERENCE:
Installing Enterprise Manager 10g Grid Control Using an Existing Database