The qeth device is configured with OSX CHPIDs. An OSX CHPID cannot be activated unless the LPAR is explicitly authorized to access it.
For z/VM guest operating systems, the z/VM user ID must be explicitly authorized in addition to the LPAR.
You grant these authorizations through the Service Element.
User response
At the Service Element, authorize the LPAR and, if applicable, the z/VM user ID for using the OSX CHPIDs with which the qeth device has been configured.
Then try again to set the device online.