When you try to install RM Unify AD Sync, the setup pauses at the RM Unify AD Sync Service Configuration Editor window and the following error message is displayed:
"specify a domain controller"

Please refer to the relevant Microsoft documentation for the procedure to install AD LDS on your server's OS. For Windows Server 2012R2, for example, the procedure is:

1. Open Server Manager Dashboard. Click 'Add roles and features'.
2. In the 'Before you begin' screen, verify the tasks displayed have been completed and click Next.
3. Click the 'Role-based or feature-based installation' radio button and click Next.
4. In the 'Select destination server' screen, select the AD Sync server and click Next.
5. Tick the Active Directory Lightweight Directory Services box and click Next.
6. Follow the instructions in the window to complete the process.