If a replication system domain has Replication Server 15.5 and later, the system version, and all site and route versions in the replication system domain must be version 12.6 and later.
You must upgrade Replication Server to version 12.6 or later, set the site version to 12.6 or later, and upgrade routes to 12.6 or later, before you can upgrade to version 15.5.
See “Upgrading or Downgrading Replication Server” in the Replication Server Configuration Guide.