Cannot open DFU device! Electron

You actually have the 0.6.0-rc.2 system firmware flashed to your device via CLI or dfu-util?

If you still haven’t got 0.6.0-rc.2 system version installed (which is a three-part upload via CLI/dfu-util - and something else than your application firmware) - then you need to do that first!
(BTW, that’s also what that message box is trying to tell you all along)

If you have, you can either download the binary and flash it via USB

particle flash --usb <yourFirmware.bin>

Or you force Build to update its device info by flashing a simple (even empty) project targeted for 0.4.9 OTA, then close and reload Build.
If it hasn’t updated to “On the device: 0.6.0-rc.2” try the CLEAR CACHE button in the settings drawer of Build.

After that you should be able to flash 0.6.0-rc.2 firmware OTA with Build too.

Here are the binaries required for the system update

And here is how it’s done
Particle Firmware Updates Thread