You need a database repository to install the OEM cloud control and a weblogic instance? Did you create any repository and weblogic instance? Are you using the default one?
You do not need to install the full OEM cloud control 12c. This needs lot of resources. If you are going to monitor just one database, you can use use "dbcontrol" (same as OEM but for only one database). You can google how to create dbcontrol. Check this link as well
I need the OEM for the administration tasks purpose! I have all the print-screens of this installation, how can I send a docx document to show you this?! I would like to post here those images of my installation for I can show you but unfortunately the "insert picture" button here is disabled (I don´t know why!).
you can do the administration with the dbconsole also. It comes with the database you already had installed. It's almost same as OEM. The only difference is you can manage multiple databases with OEM but with dbconsole you can only manage that particular database. If you just want to administer that particular database, you can just use dbconsole.
Refer the prerequistes and make sure host requirements met as per
After implemtning host requirements, re-try the installation.
For repository DB, prerequistes refer