You need to prepare your forest first.
Log on at the server which is schema master for your forest, with user name
that is member of Schema Admins or Enterprise Admins group. Run setup with
/forestprep switch.
After you finish forestprep, you may log on the domain where you will
install Exchange and run setup with /domainprep switch.
After forestprep and domainprep, you may setup Exchange on the server of
your choice in the domain that was prepared.
Dusko Savatovic