Okay, your CPE vendor has probably set Device.XMPP.Connection as not writeable. Set cwmp.skipWritableCheck to true in Admin → Config. Then reload the CPE in the UI and see if you can now add an instance. If you can, then let your CPE vendor know they have messed things up.
You need to contact your vendor to have them fix the issue. Turn on debug for the device in question, refresh Device.XMPP.Connection and then show them the soap trace showing writable as false for Device.XMPP.Connection and then point them to the spec.