Failed to create child domain under the 2nd root domain

Hello.

I have installed 2 root domains in 1 forest, the root 1 is Win2K12 Datacenter (DNS), and the root 2 is Win2K8 SP2 Enterprise x64. And I wanted to create a child domain under the root 2, with Win2K12 Datacenter, also I have been ran Adprep.exe before the installing, but cannot setup the child domain successfully.

The installation is always showing "Replicating the schema directory partition". I have no any idea for the issue...

The image.

Kind Regards.