/api/EventRegistration/_execute returns back ComboOrder error

I'm sending the following Register Event request to iMIS.

{
                "$type": "Asi.Soa.Events.DataContracts.EventRegistrationRequest, Asi.Contracts",
                "EntityTypeName": "EventRegistration",
                "OperationName": "RegisterEvent",
                "RegistrationType": {
                    "$type": "Asi.Soa.Events.DataContracts.EventRegistrationType, Asi.Contracts",
                    "$value": 3
                },
                "EventId": "EventCode",
                "RegistrationOptionFunctionId": "EventCode/EventReg",
                "FunctionId": "EventReg",
                "RegistrantId": "316292",
                "RegisteredBy": "316292",
                "BillTo": "316292",
                "Waitlist": false
            }

iMIS responds with the following 200 OK.

{
    "$type": "Asi.Soa.Core.DataContracts.ValidateResultsData, Asi.Contracts",
    "IsValid": false,
    "ValidationResults": {
        "$type": "Asi.Soa.Core.DataContracts.ValidationResultsData, Asi.Contracts",
        "Errors": {
            "$type": "Asi.Soa.Core.DataContracts.ValidationResultDataCollection, Asi.Contracts",
            "$values": [
                {
                    "$type": "Asi.Soa.Core.DataContracts.ValidationResultData, Asi.Contracts",
                    "Message": "ComboOrder error for registering party id 316292 in event id EventCode"
                }
            ]
        },
        "Warnings": {
            "$type": "Asi.Soa.Core.DataContracts.ValidationResultDataCollection, Asi.Contracts",
            "$values": []
        }
    }
}

Is there a way to dig into why the ComboOrder errored?

The registration option is set for $1,140 non-member and $995 member. No capacity set. iMIS version is 20.3.192.241