Error: trapped plant loop index = 0

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This isn't a full answer, but I've found it sometimes can be helpful when encountering an unfamiliar error to search the EnergyPlus source code using a bit of the error string to see what comes up. In you case, this error only shows up once, when (and this is kind of a stretch, since I don't know C++ that well) the plant loop index is zero.

I would double-check that you've connected your SwimmingPool objects correctly to the proper demand branch(es) of your loop. If you're still getting the error, you might want to upload your idf.

I see a crash in ZoneHVAC:LowTemperatureRadiant:VariableFlow since the cooling water inlet/outlet nodes are 0. Do you only want heating from the radiant system? If so, Cooling Design Capacity Method should be None if not used for cooling. But wait, the IDD says None is an option, but the program stops if None is used.

So the only suggestion I can make is to create a chilled water plant loop to be able to simulate this model (or get rid of the ZoneHVAC:LowTemperatureRadiant:VariableFlow object).

ZoneHVAC:LowTemperatureRadiant:VariableFlow,
  A10,  \field Cooling Design Capacity Method
    \type choice
    \key None
    \key CoolingDesignCapacity
    \key CapacityPerFloorArea
    \key FractionOfAutosizedCoolingCapacity

ZoneHVAC:LowTemperatureRadiant:VariableFlow,
 Low Temp Radiant Var Flow,  !- Name
 Always On Discrete 1,    !- Availability Schedule Name
 Greenhouse Main Thermal Zone,  !- Zone Name
 Low Temp Radiant Var FlowFloors,  !- Surface Name or Radiant Surface Group Name
 0.043,                   !- Hydronic Tubing Inside Diameter {m}
 8778,                    !- Hydronic Tubing Length {m}
 MeanAirTemperature,      !- Temperature Control Type
 HeatingDesignCapacity,   !- Heating Design Capacity Method
 Autosize,                !- Heating Design Capacity {W}
 ,                        !- Heating Design Capacity Per Floor Area {W/m2}
 ,                        !- Fraction of Autosized Heating Design Capacity
 0.02,                    !- Maximum Hot Water Flow {m3/s}
 Node 8,                  !- Heating Water Inlet Node Name
 Node 10,                 !- Heating Water Outlet Node Name
 0.5,                     !- Heating Control Throttling Range {deltaC}
 Greenhouse Heating Schedule,  !- Heating Control Temperature Schedule Name
 CoolingDesignCapacity,   !- Cooling Design Capacity Method
 Autosize,                !- Cooling Design Capacity {W}
 ,                        !- Cooling Design Capacity Per Floor Area {W/m2}
 ,                        !- Fraction of Autosized Cooling Design Capacity
 0,                       !- Maximum Cold Water Flow {m3/s}
 ,                        !- Cooling Water Inlet Node Name
 ,                        !- Cooling Water Outlet Node Name

and the err file shows this:

** Severe  ** SetComponentFlowRate: trapped plant loop index = 0, check component with inlet node named=NODE 54

and this: * Warning * SizePlantLoop: Calculated Plant Sizing Design Volume Flow Rate=[0.00] is too small. Set to 0.0 * ~~~ * ..occurs for PlantLoop=SWIMMING POOL LOOP

the bnd file shows node 54 as: Node,54,GREENHOUSE MAIN THERMAL ZONE RETURN AIR NODE,Air,0