You are getting the error because a zone defined as a return plenum cannot also have ZoneHVAC equipment serving that zone. This restriction is mentioned in the I/O reference for AirLoopHVAC:ReturnPlenum AirLoopHVAC:Return Plenum field Zone Node Name:

Do not use the Zone Air Node Name specified in any ZoneHVAC:EquipmentConnections object, and since this zone is a plenum a ZoneHVAC:EquipmentConnections object is not required for the zone named in the previous field.

Here is a somewhat similar question regarding conditioning a plenum zone with radiant heaters. Their nominal solution was to separate the plenum into two zones.zones.