installing Exchange Step 6 of 13: Mailbox role: Transport Service - Object(EXCH) couldn't be found

So your original role that the Exchange Server you are now installing was a DC? You uninstalled the DC role and then when you install Exchange Server you get lots of errors, which you resolved and now have the above error and if you resolve that you have no idea if you are going to get lots more errors?

Yes, it is all to do with the server being a DC. Start with a server installed from scratch from the installation media and maybe even just start from scratch and build a new DC from the install media - why are you still looking at SBS server - its not been on the market for years. Also there is something in the post about a clone being made - don't even go there with DC's - never clone machines and never ever clone DC's.

Uninstalling the DC role does not return the machine to the state it was in before it was a DC. It just uninstalls the role and stops it being a DC - it does not roll it back to how it was before it became a domain controller.

In the real world, if you demote a DC that is the end of its life as a server - you would be best to wipe it and start again. Therefore in a test environment or a virtual environment you need to do the same