When you run the forest prep, make sure of a couple things:
The AD you are joining, Active Directory DNS is working properly.
On your unity server, the DNS addresses are pointed to this DNS server in AD
Also, make sure the account you are using has the correct privilages. You may want to run the forest prep on a domain controller. This will help get around your problem as well. Once the forest prep is done on the domain, you can go back and run Exchange, etc.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.