This is an issue I've seen before when installing Exchange in a child domain. The problem was that the mailbox role fails the install at 98% as it cannot start the transport service.
If you try to start this service, it starts then stops and you see the below error in the event log:
The worker process crashes continuously on startup: c:\Program Files\Microsoft\Exchange Server\V15\Bin\edgetransport.exe. The service will be stopped.
To resolve this problem, you'll need to go back and ensure that you have run the AD preparation tasks correctly using accounts with the right permissions and that each command was run on computers in the correct site and domain. These are tasks are listed below:
1) Prepare Schema
2) Prepare Active Directory
3) Prepare Active Directory Domains
See my recent post on how to do this here.