My DCDIAG reports that the Active Directory Replications test has failed with error 1256 "The remote system is not available. Any body who can suggers?

- Check the connectivity between both servers " pings " and firewall settings on both servers 

- Check your DNS errors 

- Check site and services if NTDS setting between both servers is connected 

- Check your Antivirus in case you use it as firewall too which deny any network traffic 

After verifying network connectivity (Already failed),  and firewall configuration, you can  manually  execute replication or  reboot the remote server.

Dear 

kindly add the additional server then you can perform these step remotely. if you already added then check the network connectivity also check the DNS, if still not working then review the firewall configuration. 

When the destination DC fails to bind to the source DC using RPC a win32 error code in the Repsfrom status for that partition - usually Schema or Configuration since these partitions are replicated at a higher priority.  After an RPC bind failure has occurred, a cleanup routine will run to clear the destination DCs queue from that same source DC.  This is done to avoid wasting time attempting to replicate with a DC that it can't connect to.  Since it hasn't attempted a sync for the partitions that have been cleared from the queue, a status 1256 is logged.  In a scenario where destination DC replicates Schema, Configuration, and several GC non-writable partitions from the source DC, the win32 error status for the Schema and Configuration partitions that caused the RPC bind failure is logged.  The destination DC will then cancel the pending replication tasks for the remaining partitions and log win32 error 1256 for the status.