I have 4 Powerconnect 7048 with SFP+ rear modules and the cables connected as in the official documentation, but I can't stack, always having the message Unassigned.

I did these steps:
en
conf
show switch stack-ports
stack
stack-port Tengigabitethernet X/1/1 stack
stack-port Tengigabitethernet X/1/2 stack
end
copy running-config startup-config
reload

How to make it work?

Thanks