Oracle VM Manager 3.1.1 Oracle XE got error

Dear All,
Oracle VM Manager 3.1.1 uses default Oracle database (Oracle XE) is installed in Oracle Linux 5.8.
 
I try to login into Oracle database and access to sys or system user but got error message as below:

root@manager ~]# /sbin/service ovmm stop
Stopping Oracle VM Manager                                 [  OK  ]
[root@manager ~]# su - oracle
[oracle@manager ~]$ sqlplus system/Heng123456
-bash: sqlplus: command not found
[oracle@manager ~]$
[oracle@manager ~]$ sqlite3 system/Heng123456
SQLite version 3.3.6
Enter ".help" for instructions
sqlite> drop user ovs cascade;
Unable to open database "system/Heng123456": unable to open database file
[oracle@manager ~]$


Please kindly give advice for this.

Thanks and regards,