In a mixed-version environment, Replication Server must be version 12.6 and later.
If a replication system domain has Replication Server 15.5 and later, the system version and route versions in the replication system domain must be version 12.6 and later. Replication Server 15.5 does not support mixed-version environments containing versions earlier than 12.6.
See Replication Server Configuration Guide for your platform > Requirements for Upgrading to Version 15.5 and Later.