So, I can't add new themes and I can't find nothing on the wordpress forum too.
This is a multisite. You can add new themes on the network page
/wp-admin/network/themes.php only. In the My Sites menu go to Network Admin/Themes. There you will find a list all installed themes and the Add new button.
If you cannot access that page, you are not a Super Admin on the network. Super Admins are the only users who can access those sites and install new themes and plugins.