Sintaxis in function for downloading a MQTT downlink message to a device in TTN

General
2

I don't use any of that equipment nor do I use LoRa - however, looking at other posts I suspect your message structure is not right.

Try this instead...

msg.payload = {
    "end_device_ids":{
        "device_id": "eui-70b3d57ed005629d",
        "application_ids": {
            "application_id": "rfm6601-test-1"
        },
        "dev_eui": "70B3D57ED005629D",
        "join_eui": "323531322F002E00",
        "dev_addr": "260B7D71"
    },
    "downlinks": [{
            "f_port": 15,
            "frm_payload": msg.payload.toString("base64"),
            "priority": "NORMAL"
    }]
}
return msg;

(this ↑ is pure guess work - if it does not work, point me to documentation and I'll try to help further)