ReliableDR cannot be installed on a DC server
ReliableDR uses an SQL server database to persist the IT recovery data and orchestrations.
Microsoft warns about different problems when installing SQL Server on a domain controller:
It is not recommended to install SQL Server on a domain controller. There are specific security restrictions when running SQL Server in this configuration and given the resource demands of a domain controller, SQL Server performance may be degraded. Furthermore, SQL Server is not supported on a read-only domain controller. Setup will normally fail. Even if you find methods to work around the problem with setup, SQL Server is not supported on a read-only domain controller. In addition, SQL Server failover clustering is not supported to install on a domain controller. (*)
To avoid all that problems ReliableDR will not be installed on DC servers.
(*) Extended info on this article from Microsoft knowledge database.