I am having a case where I want to share a string to a BigQueryInsertJobOperator and use it as a key to take a given configuration. I try to do it as is:

Inside dag definition I am using PythonOperator to push a variable to xcom:

def capture_mode(**context):
    trigger = context["dag_run"].conf["message"].strip()
    mode = "case_a" if (trigger == "trigger_casea") else "case_b"
    task_instance = context["task_instance"]
    task_instance.xcom_push(key="mode", value=mode)
    return mode

capture_mode = PythonOperator(
    task_id="capture_mode",
    provide_context=True,
    python_callable=capture_mode,
    dag=dag,
)

Afterwards I try to pull the parameter mode to use it as a parameter key for a json configuration:

copy_scoring_data = BigQueryInsertJobOperator(
    task_id="copy-table",
    configuration={
        "copy": {
            "sourceTable": {
                "projectId": copy_data_config["sourceTable"]["projectId"],
                "datasetId": copy_data_config["sourceTable"]["datasetId"],
                "tableId": copy_data_config["sourceTable"]["tableId"][
                    "{{task_instance.xcom_pull(task_ids='capture_mode', key='mode')}}"
                ],
            },
            "destinationTable": {
                "projectId": copy_data_config["destinationTable"]["projectId"],
                "datasetId": copy_data_config["destinationTable"]["datasetId"],
                "tableId": copy_data_config["destinationTable"]["tableId"][
                    "{{task_instance.xcom_pull(task_ids='capture_mode', key='mode')}}"
                ],
            },
            "createDisposition": "CREATE_IF_NEEDED",
            "writeDisposition": "WRITE_TRUNCATE",
            "useLegacySql": False,
            "priority": "BATCH",
        }
    },
    impersonation_chain=config["serviceAccountName"],
    location=config["datasetsLocation"],
)

the json configuration looks like this:

  "copy_scoring_data": {
  "sourceTable": {
    "projectId": "project_id",
    "datasetId": "dataset_id",
    "tableId": {
      "case_a": "table_a",
      "case_b": "table_b"
    }
  }

Here I am trying to pull the right value in order to get case_a or case_b configuration.:

  {{task_instance.xcom_pull(task_ids='capture_mode', key='mode')}}

But I am getting a KeyError in the lines referencing the line above.
What am I doing wrong ?