Re: QFX3500 Configure Management Port but not me0

It is already configured for ethernet switching, so you'll need to delete it so you can re-configure it as a layer 3 interface:
 
del interfaces ge-0/0/12 
set interfaces ge-0/0/12 unit 0 family inet address 10.0.0.1/24
set interfaces ge-0/0/12 description "Management port"