RPC Issue when trying to replicate from Primary AD and newly secondary additional domain controller

I am working as an administrator in a hospital and trying to migrate Primary domain controller from Win server 2008 r2 to Win server 2016. We ran into a problem while adding additional domain controller. We reached to the replication to replicate from the primary to the secondary. "Replicate now" command give us the bellow error:

"The following error occurred during the attempt to contact the domain controller xxxx The RPC server is unavailable"

I just need to solve this issue and move on with the project.