You can try an alternative way of flashing fw from: https://www.espruino.com/Firmware+Update#nrf52
Also see this if it can help:https://www.espruino.com/Troubleshooting+Bangle.js
@Ganblejs started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
You can try an alternative way of flashing fw from: https://www.espruino.com/Firmware+Update#nrf52
Also see this if it can help:https://www.espruino.com/Troubleshooting+Bangle.js